Ok. Dangers of street drugs aside. (I contend that the most dangerous part of the X isn't the MDMA, but what crazy filler has been added to it.. Caffeine, Herion, Crystal Meth, Cocaine.. You never know until you take it..)
I've had sex a few times on X. Usually it doesn't do wonders for your erections (if you can manage to get it up), but I really had fun being a bottom! You just have to remember that you are in an altered state of consciousness, and are hindered from making good choices. I would suggest a more controlled environment (A hotel room, person's house, etc) than in public (Circuit Party, Bathouse, Sex Party). This is VERY important if you haven't done it before. Usually people rolling look like a total mess, especially the first couple of times.
I also have to mention that I've always had more fun on X if you maintain the "Touchy-Feeling" roll to the "Hard and Horny" roll. The benefits of MDMA really come out when you are in a more social setting, and I've always had more fun dancing, meeting new people (I'm usually shy), giving back rubs, etc than fucking all night long (Which is totally possible on X).
The other big thing with MDMA is that you've got to give yourself some time to recover afterwords.
"Drugs are bad..Stay away from them..There is a time and place for everything. It's called college." - Chef from South Park