I agree, if you execute Loghain, I'm pretty sure both Leliana and Zevran would fine the Warden sleeping with Morrigan once if it meant that their lover and a friend of theirs didn't have to die. That's a pretty good bargin, as Leliana would say. Besides, convincing Alistair to do it is super creepy and I would never take that option if I didn't have to. (And is why I feel bad for all female Wardens.)
I always figured Zevran would be on board with the DR (no matter who was performing it), but I was surprised by Leliana in Inquisition being totally okay with it.
More than okay, in fact. Less because you're sleeping with someone else and more because
you're fathering a child you will never see and I'm pretty sure crazy ritual sex magic isn't sanctioned by the Chantry...
But yeah, I don't let Alistair do the DR anymore. Makes me feel gross, mostly 'cause we know that he doesn't treat sex lightly, he wants it to be special and not with a woman he loathes... and since Al was a bastard child himself, abandoned by his father, I just... can't.
I'd like to see Loghain's reaction. Maybe it's on YouTube.