Trends in STDs are going to be caused by people who are having a lot of unprotected sex. So those people who "have had actual sex with another human" count for nothing. What you need is a study that shows which people have been having more sex, and why they have been having unprotected sex.
Now compare that "have had actual sex with another human" study with studies from places like OK Cupid where they found that:
- women overall got the most messages on dating sites
- the second decile of women got more than 50% of the attention (ie: the hottest women got a lot of messages but the "in my league" hot women got far more)
- most women got some attention (even just 1 message during the study)
- only the most attractive men got attention, but closer to the level of the least attractive women
- the most attractive men will "date down" to improve their chances of having sex on their terms
- women in general will "date up"
Informal study with ten invented people: http://jonmillward.com/blog/attraction-dating/cupid-on-trial...
OK Cupid has plenty of studies, but here's one that looks at who sends the first message (overwhelmingly men): https://theblog.okcupid.com/a-womans-advantage-82d5074dde2d (there's another study based on interracial relations, and it turns out that all women prefer white men).
Spreading STDs isn't about how much sex you have, it's whether the one person you had sex with had an STD. Given the extremely asymmetrical nature of the dating and hookup successes, it's highly likely that the one person you had sex with this month had sex with a lot more people than you.
It doesn't help that the news agencies are reporting "cure for AIDs" and nonsense like that, so people don't take STDs seriously — we basically have an anti-awareness campaign making people more cavalier about sexual health.
There's no real puzzle here, just lots of contributing factors.