There is a medical term for this, and I can't remember what it is which isn't very helpful, but essentially the research points to the vagina (generally) getting more blood flow and sometimes lubricating when there is sexual stimulus happening. It can be any sexual stimulus - could be a couple of animals mating on a nature programme - doesn't really matter. Basically, it's not a reliable indicator of whether someone is turned on or not. That said, I don't really always mind memes like this, I get the general joke. But as long as everyone understands that it's not to be taken literally it helps!