So people are saying “look at me, I’m selfish” as a joke?

You’re completely right that not every bad behaviour in a male is toxic masculinity. And a completely non-sexist person may just not want to do it out of pure selfishness. However there are plenty of jokes about the gayness of a man eating out a woman. I’m pretty sure these topics are closely related. The logic of those men usually looks like this:

Eating a woman out => submissive => weak => bad