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."
To answer the question implied ( how do lovers with non-hetero bits get it on? ) the answer is any way they can. The sex is not about making sure the parts fit but engaging in physical intimacy and expressing love (or at least extreme horniness) for each other, and so any given couple is going to figure something out, even if its frottage or oral or whatever.
The analogy with the plugs and sockets is so lame anyways.
They want to compare a plug and a socket but the analogy falls apart once one takes a moment to realise that the penis analogy, a plug, is a receiver for electricity.
Do eggcells suddenly travel up dudes dicks and deposit babies inside them?