MDMA itself actually does not make the user feel extremely sexual; it has the opposite effect. When I'm on it, I would much rather cuddle with a girl than have sex with her. It's been years since I've done it, but throughout my life I probably ingested around 1500 MDMA pills, and not once did I have sex while on it.
Of course, street-level Ecstasy is cut with many fillers, especially amphetamines. I'm assuming that these are the reason why Ecstasy has a reputation for "making you really horny."