If they're going to do it, there should be a feeling of it evolving... DAO got that right. DA2 didn't. At least IMO. Alistair is a good example or leliana... you feel like it's evolving to something. With Zev the sex comes first but after that you do feel it's evolving. There is a sense of intimacy there. You have some choices in dialogue, most of which will lead you to romance. That's realistic and better than the dumb heart dialogue wheel.
As for the sex scenes themselves, underwear is absurd. I have nude mods and mods that give you an actual scene. It gives it a feel.... like if you are watching a movie and they have an intimate scene that leads to sex and you are really wrapped up in it, it's pulled you in, then it all works really well. You feel the romance and passion and even love. That's a well done scene. In a game, it can be well done too, when written well and allowing for interactions that allow the player to feel the experience since it is role playing. Doesn't have to be some long torrid scene. Actually the shorter more romantic ones work the best I think. Just enough to give you a feel, not like porn, more like romance and people in love. They works well... you feel like you are in the romance and it's FUN. No weird awkwardness. Like being really immersed in a movie with a couple that just sweeps you away. When it's over you feel like you were part of it.
DA2 didn't have that. DAO did well for the most part but the scenes they gave us weren't really that well done and looked like crap. Mods fixed them so it's like you are watching a romantic kiss in a great movie. It's really just a matter of handling it in an adult manner, rather than making it like it is for a fourteen year old kid, and of course, avoiding what looks like straight up porn. There is a balance, which can be easily achieved. And if you don't like it, then you just don't do that part but for those that find it adds something to it... then it works well. Frankly, I would never feel a romance is complete without the couple having sex. Most writing (books, movies, TV) brings a romance to the point where the couple has sex because it is the natural progression of things and gives it a feeling that it has evolved into it's most mature state. When was the last time you saw a romance in the media or lived one in your life that never reached this point? Generally when they don't it is because it was not an ideal or even a good relationship. If it were, it would have gone further. We're doing a game romance for the ideal.
I suppose a couple kissing in a way that alludes to what will come next is also another way to handle it. That has been done very successfully in movies and tv. It could work well for games. It's all about how it is handled and if it gives a feeling of satisfaction. You don't need to see the scene to know what was going to happen next and be caught up in the feeling of it which is what RPGs are all about.