Preliminary studies show that X, like many drugs, works through the same brain pathways as those that give the pleasurable feelings of sex. The odd thing is that X seems to make sex aversive rather than rewarding. If you give MDMA to male rats and let them mate (they don't seem to have any trouble with that), the next time you put them with a female, they avoid the female. This is totally preliminary (unpublished), so we'll have to wait a bit to see if it holds up when replicated. Anecdotally, we've heard this is similar to what humans experience...no problem the first time, but sex isn't very pleasurable subsequently (I wasn't aware of the impotence component previously, though when getting information anectdotally, it's not always complete.)
When I heard about these effects, I was puzzled why it was so popular. You'd think those side effects alone would turn people off trying it.