see my reply to sugar_in_my_tea for more of my take on this, but yes I do agree that it is a broad generalization and ... well really, either my way or your way of interpreting it, it isn't really that funny.

... It is alluding to an old and persistent stereotype that gay men are more likely to be highly promiscuous and basically go home with a different dude from the gay bar every night.

This stereotype itself hugely played into the 'Gay Panic' during the onset of GRIDS/AIDS. AIDS was originally called Gay Related ImmunoDeficiency Syndrome because it initially popped up mostly amongst gay men, and the media and culture at the time largely presented it as a scourge from God meant to punish the gays for their evil ways.

I have known a good number of gay men, and at least anecdotally, sure I've known a few who were quite permiscuous, and they'd be the ones to make this kind of meme/joke with a degree of self aware irony, but the vast majority were monogamous and would basically be either moderately or significantly offended at anyone making this joke, and would say that the aforementioned promiscuous gay guys were doing more harm than good by perpetuating the stereotype by making the joke, even ironically.

interestingly in real life that is entirely inaccurate across the board for women and men I know. Probably has to do with the type of person I interact with.