There was a sort of pop-economics book titled after the paradoxical truth "More sex is safer sex".

It's about topology. Star topologies (e.g. one man and many women) are more spreading of STDs than graphs that have evenly distributed edges.

More total people having sex with more partners brings about fewer STDs due to diffusion.

https://books.google.co.ao/books?id=07sYZRyFXAAC&printsec=fr...

People aren't scared of STDs like they used to be. Even HIV isn't considered scary any more. Couple that with how good birth control is and easy access to abortions and you find unprotected sex is the norm now. For high levels of STDs you don't need people to be having a lot of sex; it only takes one time to catch something. So promiscuity and low condom use would account for it.