Spoil me please, Ser Gilmore mod and romance...

You can have Alistair or Loghain do the Dark Ritual with Morrigan rather than having your PC sleep with her. Also, none of the other love interests break up with you if you sleep with her for the DR. I know that Ser Gilmore is extra picky but I would think doing the DR with your PC wouldn't cause him to break up with you although I could see him frowning on the Dark Ritual being done at all.

So I did some testing. I've only romanced him with a female, and I know he's very pro dark ritual when you play a female warden and you need a persuasion check to keep the relationship going if you refuse.
Was not in the mood to replay the whole game with a guy, so used some dark magic to trick the game into thinking my Warden was a lad. And Rory is not happy if the Warden does the deed themselves and wants a persuasion check to keep the relationship going. He does approve if you convince Loghain (and presumably Alistair as well) to do the ritual.
Also am assuming he gets also gets huffy and wants a persuasion check if you refuse the ritual all together as a dude.

So basically, you're going to want to invest some points into persuasion.
Was too lazy to check on the proposal, but since my lady who refused the ritual and persuaded Rory got the proposal, I'm guessing you'll be fine if he doesn't dump you at the gates.

At least, that's how it played out in my game. But I was testing on an old save so it's not 100%, but hopefully it is mostly correct.