She will offer the DR to be done with Alistair if you didn't romance her (I have a couple of playthroughs like this), but I am unsure if that option is available if you romance her.
Sending the woman I loved to have sex (and a baby) with another man (one she despises) makes me wonder why he would do the romance at all. And would doubtless create a ton of resentment from his lover. Possibly ending the romance I would think? Seeing as the player's motivation is to use the child for his own purposes (possibly counter to Morrigan's who will flat out tell you she doesn't intend to create an heir, but has her own use for the child) might make this not a feasible outcome to the game.
I'd say easiest thing for the playthrough, both in terms of gameplay AND story logic, is go for the sexy elf and just stay BFF's with Morrigan, and force Alistair to do it.
However, I am unsure if the option to go with her into the Eluvian is available if you don't father the OGB or romance her.
Yeah, I know the option to have Alistair to do it is still available if you're a male and romanced someone else, and when I romanced Morrigan in the past I always jumped to do the ritual with her. Unfortunately, I don't have any of my old game saves to check. I also did a search before I posted (using Google), and came up empty handed.
My concern is similar to the one you pointed out. As you advanced through Morrigan's friendship and / or romance she starts to let her armor down. She develops legitimate feelings for the Warden. This makes things more complicated for the relationship, as she might not see the forest for the trees, and my character probably won't be given dialog options to describe the obvious benefit of having Alistair first born child in their possession.
My Warden is not a good guy. Character wise he's more on the same wave length as Morrigan (pre-friendship) and Flemeth. Morrigan is the type of character in my view who would respect and admire my Warden's reasoning for wanting Alistair to do the ritual. Oh, she may protest, that she doesn't NEED or WANT an heir to the throne. However, if I had free reign over my Warden's dialog he'd respond with something like: "I see. I wasn't aware that foretelling the future was part of the magic Flemeth taught you. Never turn your back on gaining a potential advantage over your enemies, my love. Regardless of whether or not you intend for him to sit on the throne is irrelevant, it's having the option available to you should you desire it in the future that's important. If nothing else, it is useful leverage to use against Anora and Alistair should it become necessary."
At which point, I would predict that Morrigan would protest, claiming Alistair would never agree to sleep with her. My Warden would respond to that by pointing out: "He trusts me. I've put enough on his plate to ensure that his mind is thoroughly occupied with being the future king of Ferelden at the moment. That combined with the impending battle against the Arch-demon weighs heavily on him. Alistair is naive to the hard ways of the world, and he won't think things through completely to their future implications. I will play to his sense of honor and duty, reminding him that Ferelden needs it's king - now more than ever. And if he thinks about it overly much, and can't be convinced through normal means I can always fall back on my blood magic."
Finally, I imagine Morrigan giving one final protest, as my Warden is getting up and already leaving the room. He's basically made the decision for them both, and Morrigan is uncertain - reluctant, but not finding a way to get out of it. She tells the Warden that she doesn't even like Alistair, that she can't STAND him, and that if she has to do the ritual with anyone, she'd want to do it with the Warden. To which my Warden would respond (as he's walking out of the door to get Alistair): "Ah, my sweet. It will be over before you know what's happening. It may be the worst fifteen seconds of your life, but it will be worth it in the end."
I'm pretty sure at that moment, Morrigan would retort about what happens if it lasts longer than fifteen seconds, "what if it lasts thirty seconds". To which my Warden would respond: "Then I will go down to the Chantry and pray." They'd then both laugh at the ludicrous notion that my Elven Warden would be caught anywhere near a Chantry unless it's to burn it to the ground. ...and he'd be gone to get Alistair, even over Morrigan's protests.
My Warden isn't nice or good by any stretch of the imagination. He was born in the Denerim Alienage, and still has clear memories of what his life was like there. How Elves outside the circle were treated - how his mother and father were treated. He remembers being ripped away from his family at a young age by the templars when he began to show signs of magical abilities. His father told him, when they started to develop, that he had to be careful or it would happen. That the shems would never accept an Elf with more power than them. His parents were actually proud of his talents, but were fearful of what would happen to him. He was ultimately taken, and they were slain by the templars as they tried to protect him.
At the tower he hardened his heart, and swore vengeance against the templars, the shems, and the Chantry as a whole. He suffered further indignities as a tower mage both from his fellow (human) mages (both apprentices as well as full mages) and the templars. It was a hard lesson for a child to learn, but it taught him that he had to keep his head down, suffer what he must, for as long as he must, until such a time would come to pass where he was powerful enough to get his vengeance. He threw all of his ability at his training, and became the best and most promising student that the Tower had seen in generations. He had to work harder than the humans to prove that he was capable. He completed the Harrowing in record time, the quickest that anyone in the Tower has ever completed it.
Then when it came to his friend Jowan, in my head canon, my Warden had a secret romantic crush on him. However, he knew Jowan would never return such feelings so kept it to himself. Then he finds out about Lilly and Jowan's plan. Well, it came down to whether or not my Warden (then just a Circle Mage) would risk everything he'd worked so hard for in an effort to save the guy he had feelings for and his girlfriend. In the end, after probing Irving for a bit to see how much he knew of the plan, and realizing that Irving already knew too much for it to be successful he betrayed Jowan and Lilly. He conspired with Irving to get Jowan's little Chantry b*tch, and make her suffer since Jowan will also suffer as well. His heart hurts as he knows Jowan's fate and basically leads him right to the headsman, but by this point my Elven Mage is hardened to the point where he's willing to do whatever is necessary to further himself. Even if it means sacrificing the people he loves.
In the end, that's what happens. Jowan of course is revealed to be a blood mage and escapes the Tower, to which my soon-to-be Warden is very pleased with. (He is not in the least bit upset by the betrayal. He's more upset that he didn't know the truth sooner, so that he could have tapped into Jowan's knowledge of blood magic. ...though, in truth, had he known he probably would have sacrificed him even sooner due to the risks involved. When Duncan offers the chance to become a Grey Warden my Elven Mage jumps at the chance. He can't believe his luck. Irving, of course, backs him up and throws his full support behind him. My Elven Mage sees this as his opportunity to finally get free of the Circle, and that's when his story truly begins.
Of course, at that point he didn't really know anything about the Joining and the fate that Grey Wardens have to suffer... that complicated things considerably, and has not made him pleased at all. His freedom came with a price. A price that he would have paid a hundred times over, but one he'd like to escape from if at all possible. That's the reason he left Avernus to continue his experiments at Soldier's Peak. He would help Avernus with his unethical research into the Darkspawn Taint if he could, forcing others to go through the Joining against their will, all in an effort to get test subjects. He would do whatever is necessary to learn Avernus secrets in how he unnaturally extended his life as my Elven mage fully intends to do the same.
Naturally, my Warden has turned to the use of Blood Magic. He was able to intimidate the demon inhabiting Connor's body to both leave AND teach him blood magic.
Anyway, that's just some insight into the mind of my Warden. Would he truly love Morrigan? I don't know. He may have feelings for her, as he isn't a psychopath. However, he's hardened enough to the point where he wouldn't be above using HER feelings for HIM to manipulate her into doing what he wants. In fact, I imagine that's how the relationship develops. The moment she tells him about the mirror that she had as a little girl, he sees it as an opportunity to find a similar such mirror, to win over her affections. It's probably both an intent to get closer to her for romantic reasons, as well as knowing that as he gets closer to her she becomes easier to manipulate and use to his own ends. Would he sacrifice her in the end, if he had to? Yeah, definitely. He's horribly pragmatic like that. He doesn't expect to survive his conflict with the Chantry and the Templars, dying in the end to destroy them.
It's best to understand my Warden through the lens of being Unfettered. His number one goal is to get vengeance on the Templars and the Chantry as a whole, and to a lesser extent humanity. Though he's less concerned with seeing humans massacred in the streets than Elves rising up to achieve their rightful place as the equals or superiors to humanity. However, he would sacrifice even that goal for his own people to burn the Chantry and the Templars to the ground - leaving them to suffer in the Alienage as second class citizens and servants to humanity. He understands the history, and what the Chantry did to his people with their Exalted March. He sees the Chantry as the source of all evil in the world, period.
Sure, they may put on a facade of righteousness, goodness, and piety. He isn't fooled by that. They brought ruin to his people through their Exalted March on the Dales. They field an army known as the Templars, which they use to intimidate nations into doing their bidding, or face the consequences. It's because of them that his parents and his ancestors were born into crippling poverty in the Alienage. They then used their templars to murder his parents and hold him a hostage for the bulk of his life, all the while teaching him that his magical abilities were a curse from the Maker. Telling him that he was always just shy of becoming an abomination, that his very existence was a danger to the rest of the world. And so, in many ways, the Chantry created a self-fulfilling prophecy, and my Elven Warden was forged in those hateful fires of prejudice and bigotry. In the end, at night, the only thing that keeps him going is his hatred and his commitment to his ultimate quest for vengeance. Everything else is second to that.
He'd even let the Blight spread across the world, tainting everything, if he thought it would serve his ultimate ends. Since it doesn't, and it stands between him and his goals, he is committed to stopping the Blight. Although, at the same time, he's actively looking for opportunities to position himself against the Chantry and the Templars. He looks for opportunities everywhere, and the Old God Baby is one such opportunity that he wouldn't pass up.
It's unclear what Morrigan's ultimate goals are, but it's difficult to imagine them differing so radically from my Warden's as to be incompatible. They both share a dislike of the Chantry (though my Warden hates it far more than she does). They also both believe that Mages should be free to practice magic. To that end, whatever Morrigan has planned would likely fit into my Warden's larger goals. He's not afraid to think long term, and he isn't fully committed to having the OGB sit on the throne. He simply wants the opportunity to be there, should it be necessary in the future.
In the bigger picture, the larger story isn't even so much about the Warden as the upsetting of the balance he ends up creating. In my head canon it will be the Warden who finds some subtle way to corrupt Anders and Justice - he's the one who plants the seed that gives rise to Justice becoming Vengeance. Which ultimately leads to the events in Kirkwall.
In Dragon Age 2, Anders will meet and fall in love with Hawke, who is an apostate mage. Someone who also hates the Chantry and the Templars, who will ultimately aid Anders and go on the run with him.
All of this ultimately comes to a head in Inquisition with the Mage vs Templar conflict. All of it because of the first ripples set off by the Warden. So regardless of whether or not the Warden even is still alive by the time it happens, he's very pleased with what he's set into motion. Naturally, my Inquisitor will be a mage (either Elven or Qunari - I haven't decided), and he will be the one who will try to finish the job.
However, all of the larger conflict has it's origin and root in my Warden's hatred of the Chantry and Templars. I'm hoping that what is happening with the Fade is a result of the Old God Baby, Morrigan, or both. In which case my Warden is EXTREMELY pleased, and it made everything worth it just to take out the Chantry leadership.
So, yeah. This is the story I'm setting up for Inquisition.