I've seen this used as a theme in someone's background (uncertain parentage, and details would be a spoiler of characters other than my own, so we'll skip them). I liked the way it was done; no explicit mentions of sex but some very nice exploration of lots of stuff. Likewise what I have seen about the breeding program stories Matariki mentiones, I have liked; tastefully done and seems to respect the topic.
I have to say though that I have a personal genuine dislike to the "I am a sex slave of So-and-so!" RP though. When it seems clear that it is done for the personal, partly sexual, enjoyment of the players involved, it seems to me to belittle the actual issue.
Again, to go back to the original thread; what "goes too far" is less about the subject matter but about how you use it.