I think it goes without saying that this can be an iffy subject in roleplaying games.
There’s a way to do this thing right, and a way to do it wrong, and more often than not it’s the latter – looking at you, bard-types who have to seduce everyone!
I try not to include it too much in games. I’ve run NPC relationships just fine, but where it interacts with players it starts to get difficult, and in danger of not being fun.
I’m sure that there are very good ways to run it, and it’s something I want to look into for ideas down the road, but I don’t think it’s something I’ll be looking at soon.