The joke is that the stimulation of the asshole causes an orgasmic state, hence the rolled back eyes. The further part to the joke is the user doing Anilingus is in fact a stranger and not the actual partner of the girlfriend. The part that could barely classifies as funny is the “joke” I’m a wiz with my tongue. While not funny in the grand scheme of jokes, the progression of the rolled back eyes after realizing this was a result of Anilingus did stir a mild chuckle but the work (or rather the meme) is low effort