I'm laughing at people laughing at that. It's the adult equivalent of toilet humor. Very little effort in that one.
The guy on the right might have said, "I actually have a B.S. in [insert biological science field here] and while I respect your right to express the gender you identify with a number of scientific treatises on the biological nature of gender have shown that, with the exception of some biochemical ambiguity that exists with certain syndromes, gender is largely defined by presence of specific hormones and hormone producing glandular cells... but YOU GO girlfriend!"
Now that'd been hilarious.