Romance
is modeled in the game, though, already, see. I guess there aren't jilted lovers in the game, yet. But, a dwarf who fails to attract a mate by being a social klutz will fail to get happy thoughts from their marriage (and sad ones as well, if something bad happens to their loved one).
Mechanical reproduction is modeled in the game as well, to a surprising degree. There is animal husbandry, and egg-layers require a different strategies. And then there are the night trolls, who can only reproduce by capturing a creature of opposite sex from another culture, converting him or her into a night troll, and breeding with him or her. That's a pretty complicated life cycle for a creature in a game, but the reason it's included is that has huge storytelling potential.
I guess other people want to see more sex in the game, too, since we have, e.g. the Succubus Dungeon mod. I haven't played it but it says it has "multi-lover" support.
http://www.bay12forums.com/smf/index.php?topic=124135.0 As other posters have said, it's all about what stories you're trying to make possible, and who you're trying to entertain. I think there is clearly a line, though, where too much graphic sex would turn some people away from the game. But there's also clearly room for a few more tiptoes before the devs cross it.