> I fear that we have a risk budget, so when STD risk is reduced via a cure or prophylaxis, humans increase the amount of sex they have until the STD rate stabilizes again.

Nope.

Gen Z rather famously is not having a lot of sex despite all the scary STDs not being as scary as they’ve been historically.

AIDS was killing over 40,000 people per year in the US at its peak. It’s now almost 10x lower despite a significantly larger population. I don’t think there’s some other STD killing tens of thousands per year to fill that gap.