I think it's a combination of the fact that our society has historically had a very perverse relationship with sex, in the sense that it is simultaneously icky, taboo and not to be talked about, yet also crucially important for brownie points and social cred.
However, the main reason is simply the fact that it's such a captivating example of unintentional humor, and what happens when you don't pay attention to your image. In many ways, it's the linguistic equivalent of the famous rabbit-duck optical illusion. It's curious and elicits a response when one realizes the double entendre.
It's hardly problematic, unless your definition of problematic is for everyone not to be dull as cardboard to every little example of unintentional humor in their lives.
