I've seen this a few times, but how the hell is this even supposed to work? Like, from a physiological standpoint, this seems completely impossible. I know it's fiction, so there's some leeway for unrealism, but surely this pushes it too far.

There's only one person I'm aware of that can do that and she's Chinese, don't know here name unfortunately.

MergedAmigo said in comment #2589240:

I've seen this a few times, but how the hell is this even supposed to work? Like, from a physiological standpoint, this seems completely impossible. I know it's fiction, so there's some leeway for unrealism, but surely this pushes it too far.

The goat's a demon. Different race, different rules.

A more in-depth answer would be that the Goat is a succubus. Succubi possess the power to alter their bodies to make the desires of their victims into reality.

Tl;dr - Sex demon magic.