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...