How interesting :)
I know when I recently let Alistair perform the Dark Ritual, I spoke to Morrigan afterwards and she talked about it. Not in any specific, "I can make you jelous as I have also lain with Alistair" details, but as I have high approval with her it was more sentimental, if you can say that. She was basically thanking me for letting her sleep with him for this is something that she wanted. Not Alistair, but the babe to come with the essence of an Old God. I did not take her to the final battle though, she asked me why and my Warden got to reply, "Due to your condition" :p
I do not know exactly how this is a Dark Ritual though, as you do not see or mention anything else, but Morrigan needing to sleep with Alistair due to his freshness to the Taint. As far as I am aware, she performed no spells, charms or drank anything. How does Morrigan know she will become with child straight away? Is she that Fertile? Makes me hope they only acted the once and not several times during this so called "Ritual" :/
I know its disappointing to realise that if you want Alistair to live with your Warden, then this is probably the main route. With my Mage Female Warden, Aria, I made it so Alistair performed the Dark Ritual, but then stayed as a Grey Warden. What was bizarre was when I mentioned about sleeping with Morrigan and a child involved, he kept going on about being king, and I reloaded several times thinking I made a mistake. I even went back through the Landsmeet. I kept thinking "Hang on! I made Anora Queen, where has this bravado suddenly come from?" Looking on the Dragon Age Wiki, its a bug! He will declare King, no matter what was said at the Landmeet, but after the final battle and in dialouge, the choices you made stay true.
I do find it hard to let Alistair spend the night with Morrigan. Surely she could have got some other way of getting his little boys without having sex? Turkey basting? Can I say that on here? I just have! Send Alistair off to the corner of the room with drawings of his beloved or something, rather than the actual sex itself. It does just seem mean, as they both hate each other and then the thought comes into my head, "those that hate each other, secretly like one another" and I then think, what if he truely had an amzing night with her? Its when Morrigan says, "you will enjoy this more than you think", that is time to skip the scene!
I suppose the only way to truely have a happy ending, even though we all love Alistair, please do not hate me by saying this, is to have a romance with one of the other characters, then when Morrigan proposes the Ritual, you do not need to worry about your feelings as much. Though then, Morrigan will have been his first (if you had not already slept together and then decided to end it). Even if I am not in a romance with Alistair, he is my Warden's best friend, so I cannot bring myself to have him scarifice or go mad with drink.
Bioware really did go out with all the endings. Who can end up with who, Where people go and what they do afterwards. Its truely amazing, and I spend hours thinking, talking and playing Dragon Age. I am a woman obesessed!