I found the ultimate sacrifce ending to be as difficult and painful to accept as doing the dark ritual, because the one you love ends up utterly destroyed by your death. Alistair breaks down and runs off and disappears because he can't take losing his girl, if you didn't make him king. If you did, it's only the kingdom and his advisors really keeping him from doing something stupid with grief. If you romanced Zevran, Zevran ends up returning to crows and never loving anyone else again. He, too ends up broken inside, a lonely, sad man.
Leliana also has a very sad ending if you die. Though the epilogue is open to interpretation, she, like Alistair, ends up disappearing, and the way it's worded makes it sound like she went off and committed suicide.
Anyway, what I'm saying, is that you can take the ritual as a good guy, as a way of sparing your love some very tragic, sad fate or life. After all, though your character doesn't know this will happen, my guess is that he would not be happy at all if Leliana ended up killing herself from grief because she didn't want to live without him.
Of course, I understand your reasons as well for not taking the ritual. After all, it is a big question mark. For one, you can't be sure on whether or not preserving an old god is a good thing or bad thing. Nor do you know what exactly Morrigan plans on doing with said child, or what said child will be like. Or what future consequences such an action will have. It depends alot on how your character view Morrigan and how he gets on.
And of course, there is the fact that you are sleeping with another woman, even though it's purely business. While Lel might be willing to accept it if she knew it would save your life, it seems your character would have a hard time coming to terms with it.
I found it the hardest damned choice in the game. Bhelen vs. Harrowmont? Coin flip. Don't care either way, both are losers. Elves vs. werewolves? Easy. Make Zathrien see what an ass he's been by beating some sense into his skull. Situation takes care of itself. Mages vs. Templars? Mages FTW. Chantry needs a good ****-slap, anyway. Defend Redcliffe or abandon it? Extra xp, loot, minor plot bonuses, and a good feeling. Plus, Teagan asked me. I always do as Teagan commands. Bann Sexy is awesome. Kill Connor or Isolde, or get the circle to help? Depends on the character I'm playing. No brainer there.
But give Morrigan demon/god child? No matter what playthrough, that part always gets me. The next choice I find difficult is ahrden/unharden Alistair and thus, who do I put on the throne?