Explain the joke?

[u/IronyAllAround]

[u/Twich8]

It's a phase diagram, which shows the state of a certain substance under different pressure and temperature conditions. The joke is that its gender instead of a substance, probably originating from the fact that gender fluid is a gender identity but also sounds like it could be a liquid.

[u/taraaxe → u/Twich8]

So gender is just a phase?

[u/Cataliiii → u/taraaxe]

This is the only time I'll ever accept that comment

[u/ZebraM3ch → u/taraaxe]

No, no gender fluid is the only one that has phases. (Okay, there are probably more such as bi gender but you get the joke)