I think the main issue is adventure mode, not fort mode.
It's easy to do all kinds of things "under the radar" in fort mode, but one of DF's major draws is that anything any NPC can do, you can do as well. (It doesn't work like that now, but that's the vision.) So if there's sex, the player must be able to initiate it. If there's rape, the player must be able to do it as well.
Since romance up to and including sex is considered acceptable in games currently (the Sims had it, right?) I think that they should be possible in adventure mode, but rape adds very little positive value to the game (gameplay-wise, stopping a rapist is conceptually not much different than stopping a thief or a murderer) and would be extremely controversial, so I don't think it's going in regardless of what anyone here thinks. DF sex will probably just be a thing that can only occur between two consenting adults, initiated through a dialogue option, and that's all there is to it.
There might be some gray areas with coercion though, depending on how logically NPC AI behaves. If you can coerce an NPC to yield, drop their items, or give in to terms of surrender by threatening them with violence (or applying violence and then threatening with more violence), could you also force someone to submit to sex in the same way? Of course, they could simply be programmed not to, but this might be glaringly unrealistic if you can coerce them to do anything else.