Can you rp sex in the forums?
To my experience, sex is best RPed as a series of experiences and sensations-there is some ruling regarding the gross physical mechanics of the act, but tiny phrases like "His tongue explored further, driving her to spasms of ecstasy" is acceptable, while more graphic descriptions are not. Try and keep it PG-13, and you should be fine. However, a thread heading warning it's fairly graphic is always appreciated, or perhaps using the post title for such a warning is warranted.
Of course, I am not a moderator, only someone whose posted sexual scenes without so much as a warning.
Also, anything illegal (ie rape fantasies, sex slaves, underage) is right out.