Most girls have boobs, and using them to represent two OOs isn't uncommon. Most girls don't have dicks, making it unexpected, surprising, and a bit taboo(which automatically makes things hotter/more fun). We're so used to boob jokes on women and penis jokes on men, but not penis jokes on women. Novelty is required to find something funny, and girls with dicks are more novel. It touches on how we aren't yet used to the bodies of some people, making this a joke on ourselves for some people. That's a bit of a reach though.

The joke is mid, and this meme is more entertaining by virtue of imagining genitals, which is a turn on for a lot of people. We all have genitals under our clothing, we just don't think about it sometimes. Most hands you shake have held a penis at some point. Sexual topics are more mundane than people realize.