My, this thread has had a long life!
Anyway... 2D6 appears to be the main enzyme that breaks down MDMA in the liver. That MDMA is also broken down by, say, 3A4 (and that 3A4 is inhibited by various common antibiotics) doesn't leap out at me as a major risk simply because 3A4 seems to be a relatively minor contributor to MDMA breakdown. So, even if you inhibit it, your body will mostly be breaking down MDMA just as it normally would. A medication that strongly inhibited 2D6 would be a much larger cause for concern.
THAT BEING SAID...
If you are on an antibiotic, your doctor thinks you either have an infection, or are at higher risk for an infection for some reason (such as a recent surgery.) A night of partying on MDMA is definitely not good for your immune system, and (under those circumstances) could lead to either a new infection or a worsening of your current infection.
For that reason, I advise against the use of MDMA while on antibiotics. Relax. Life is long and strange, and you can always indulge another time.