I love jokes like these, other variants I've heard are
"Libtard can you even tell me the difference between Sex and Gender if they're REALLY two different things?" "Well it's quite simple, you see the difference is that I didn't have gender with your mother last night."
"There's only two sexes!" "You're right! There are only two sexes, the sex I have with your mom, and the sex I have with your dad."