I don't feel that all of the LIs are inclined toward marriage though. Dorian for example would not have a positive view of marriage, so might initially refuse, laugh it off, or something along those lines (we actually had a discussion on this in the Dorian thread).
So then you run the risk of having the NPC reject your proposal. They certainly should be able to do that because I want them to have agency apart from the player, but it would REALLY suck to be rejected that way. I suppose the best alternative in such a situation would be for the NPC to say "Aw, that is so sweet... but, let me think about it."
Honestly, I don't really think this option could be done very well.
Honestly, I don't either. But you can't offer it to one LI and not others (and Cullen already has one of the most developed romances---speaking as someone with a character over 15 hours into that LONNNNGGGG dry spell between Solas's second kiss and whatever other romance content there may be without even the option to kiss him at will). And everyone's assuming Cullen's inclined to formal marriage. He may not be. The very idea may make him break out in cold sweat and run away. Just because people are calling other people their wife or husband doesn't mean they're formally married. A lot of couples in common law marriages refer to each other as wife and husband. I frankly don't care that much, I just don't see why people can't be satisfied with headcannon. I personally considered his question about the future to settle the fact they were staying together. If in your world, that means marriage, well, there was your proposal (which was entirely in keepng with his personality.)
On a less divisive subject:
I just had a really charming friendship conversation with Cass on my Dalish warrior (I've said before I think they're sisters, never mind the different races
,)
And has anyone actually found all 48 missing copies of Hard in Hightown? I found 41, gave up and pulled up the Wiki, but they only had 3 of the ones I haven't found. One of those three, I apparently can't get to until more repairs are done. The other four have me completely stumped.