So, I freely admit I've probably taken a very liberal interpretation of the DR and my Warden's relationship with Morrigan, but sometimes even I view things through rose colored glasses.
Or it could just be Morrigan's manipulative ways.

Either way (and it's a bit long):
"There is no love without sacrifice."
“When the time comes, mine will be the hand to slay it.”
“Lass, you’re young yet, there is no need for you to sacrifice yourself”
Fast on the heels of Riordan’s response to Marlana’s declaration was his own, “What?! How can you possibly say that, Lana?”
Intense sapphire eyes regarded the two men, “In the days to come, Ferelden will need its king. We didn’t do what we did to plunge the country back into war, especially since we don’t know if Anora is still alive. And it will need a senior Warden to help with the aftermath recovering from the Blight.” That scene played over and over in his head as he paced back and forth in the room that he’d been given. He wasn’t surprised that this time Eamon had separated him from Lana after the shouting match they had back in Denerim about the two of them. He’d gotten angry at the look of surprise on Eamon’s face when the Arl realized he wouldn’t be able to get Alistair to do what he said. But after that “conversation” he realized why Marlana had been so furious after Eamon had pulled her to the side for a private talk. So the separate rooms were a petty revenge.
When he’d sought out Lana after that disastrous conversation with Riordan and the truth of why it had to be a Grey Warden to kill the archdemon, she’d been nowhere to be found. None of their other companions had seen her either. Except for Morrigan, who had also seemingly vanished. The worry that the apostate was finally showing her true colors by betraying the woman she supposedly called friend and sister made his already churning gut twist up even more.
He was so worked up that when the knock came at the door, he flung open the door without asking who it was. Seeing Morrigan there instead of Lana was a brief moment of consternation before his ingrained dislike reared its ugly head, “What do you want? And more importantly, what have you done with Marlana?”
While her yellow eyes glittered with anger, he didn’t get the expected comment. “We merely talked, she and I. Which you and I need to do now.”
“Really,” the sarcasm dripped heavily in that one word, “What could we possibly talk about?”
One sharp nail jabbed into his chest causing him to step back, giving the witch enough space to enter the room and close the door behind her. “I didn’t think you wanted anyone to hear your precious Warden secrets.”
Alistair stepped back further so she wasn’t in his personal space anymore, his jaw was clenched as he spoke, “And what about them? Been eavesdropping again?”
“No. I have been reading Flemeth’s grimoire.”
He crossed his arms against his chest, “This should be good.”
“Before I go any farther, you need to read this.” A piece of parchment was shoved into his face. He scowled at her before taking it, and felt the breath hitch in his chest when he saw that it was sealed with blue wax and the Cousland crest rather than the double griffin seal of the Wardens in red wax as Marlana normally signed things. Ignoring Morrigan’s sneer, he opened the parchment to see a brief note written in Lana’s elegant hand.
Alistair,
Please listen to Morrigan with an open mind.
Lana
Frowning he looked up at the witch, “So what do you have to say that Marlana has to send a note rather than be here herself?”
“Because for some unfathomable reason she feels this is a decision that you need to make for yourself.”
“And that would be?”
“As I told Lana, I have a way out, that no Warden need die against the archdemon when you face it in the next few days.”
“It has to be something particularly gruesome if the Wardens of the past haven’t used it. What do we have to do? Sacrifice babies and virgins?”
Morrigan’s expression tightened, “No, one has to be created with just enough tainted blood to act as a vessel for the soul of the god that once was.”
He desperately wanted to sit down, but didn’t want to give Morrigan the satisfaction of seeing him in a moment of weakness. He knew he wasn’t going to like what she was going to say, but was going to listen to anyway. If there was a way to keep his love from sacrificing herself… “I think it’d be best if you start at the beginning and explain to me what you have in mind.”
She studied him for a long moment in silence, for once it wasn’t mocking, but thoughtful. In a surprisingly soft voice, she began to lay out the details. How a child could be conceived with enough tainted blood through a ritual. And that the child could be used to hold the soul of the old god when the archdemon was slain by a Grey Warden instead of the Warden dying with the dragon. She finished, “For the child have that taint, requires a Warden parent. Riordan’s Taint is too strong, and since don’t know when the Orlesian Wardens are going to arrive…”
“WHAT? Are you out of your frigging mind? I’m not sure which I’d rather do, die from the archdemon or sleep with you?”
Morrigan grabbed the collar of his shirt and pulled his face down so she could glare into his eyes, “You arrogant fool! Do you truly believe that Marlana would allow you to sacrifice yourself? After she refused to take you to free Anora from Howe? You and I both know that she’d just knock you out again. Possibly here in Redcliffe where you’d be safe until everything is over.”
She let go and shoved him back as he started to shake his head, “And if somehow she was convinced to let you go, do you want to think about what your death would do to her?”
“What do you mean?” He asked in a tight voice.
“’What do I mean’ the fool asks? I still do not understand what she sees in you, but even I know that your death would be her ending.”
“She’s not the type to commit suicide.”
Surprisingly Morrigan’s shoulders slumped and for the briefest moment looked…sad, “Not knowingly. But if you died in a way she could prevent… You and I both know that she’d slowly carve away her own soul in recrimination, that by the time her death did come it would be an empty shell that died since the woman we both care about would have died a long time before.”
She straightened and glared at him again, “And yes, I do care about her. She is the first person I could ever consider as a friend and is my chosen sister. While I want to redeem the soul of the god that once was, I want my friend to live. Moreover that she be happy.” Her pretty features twisted in disgust, “Sadly for her to be happy, you must live. Otherwise I would have paralyzed her so that she’d be safe and you’d get to do your noble sacrifice. Is it that much to ask that you sacrifice some time and seed so that she might live her life with you?”
He listened to her diatribe in silence with hard eyes and an expressionless face. “And then when the child is of age you use him against us for the throne?”
The sorceress recoiled as if struck and looked ready to flay him alive, “Fool”, she hissed. “I have no desire for such things. As I said before, I wish to redeem the soul of an ancient being and my friend to live. As I promised Lana, so I will promise you this. When the battle is over, I will leave Ferelden never to be seen again. And in return neither you nor Marlana shall follow me and I shall be allowed to raise the child.”
“And what about the soul of the unborn child?”
For the first time since that horribly awkward conversation started, Morrigan looked shocked at Alistair’s question. Just a very small hint of respected colored her tone, “Odd that you should ask the same question she did. As I told her, the child, if you wish to call it that, is soulless. It is an empty soulless vessel awaiting the spirit of the archdemon.”
It was possibly one of the most revolting things he’d been ever asked to do, but. But he could see all too easily in his mind’s eye Marlana’s devastation at his death, how after time the woman he loved would be dead, leaving only ashes of that fiery spirit that he had fallen for. And knew that while he might have the strength to go on without her, he knew that whoever did end up his queen afterwards would always be compared and found lacking. And that a large part of his soul would die with Lana.
And if Riordan was right, and the Warden’s spirit was obliterated with the archdemon’s… No, he couldn’t bear the thought that when the Calling came and with it his end, that she wouldn’t be there waiting for him…
“All right, what do I need to do?”