But, really, the STD rates will always be lower among the boring “square” population than the adventurous, promiscuous, one.
I’m sure this will be about as popular a soiled diaper here, but “promiscuity” and “adventurous” sex is mathematically welded with risk. Monogamy, and vanilla sex are essentially risk free (barring a botched blood transfusion or something along those lines)
People who want to have lots of sex, will still feel those desires in a monogamous relationship. Monogamy might make them less likely to act on those feelings, but it will make them less likely to test regularly, and stop them from being open with their partners about risks, and prevents clear communication about establishing safe sex practices.
If my partner has sex with someone else, I want them to feel safe enough to tell me. That way I can make decisions such as using protection with them that I might not otherwise, or getting tested myself, which I would not be able to otherwise.
Except you cannot guarantee the other person will actually be monogamous. We saw this with the initial AIDS epidemic in the '80s/'90s, when uber-square supposedly-monogamous (and supposedly heterosexual) individuals turned out to be anything but - to the shock of their official partner.