I have a few theories of my own, just guesses from experience.
STDs are largely about one's immune system. Most of them can be beaten back by one's own immune system if it is strong enough. HIV was so deadly, because that;s how it attacks. Condoms reduce HSV & HPV transmission by about 50%. So everyone who has had sex about twice has been exposed to them. But we don't all have those diseases, because our immune system has overcome them.
I've had FB relationships with girls who have lay counts well into the triple digits, never used a condom, and been fine. I'm sure my "d!ck brother" count is in the thousands. I was married to a woman with HSV2 for seven years, never used a condom, and never got it. I should have every STD, but I don't. There is more going on here than infection rate statistics. I think immune system is the key.
The best you're going to do with a girl is one who #1 is not fvcking anyone else, #2 likes you enough to tell you if she thinks she has anything, and #3 is not a complete moron who will not notice she has an STD. Right now my sex life is two, sometimes 3-4, house wives who only see me and I don't use a condom with, because I am their only other guy. I also sometimes fvck another girl who I know fvcks other guys, so I use a condom with her. I have a 21 y/o girl living on my couch right now. We used to fvck, but after learning what a huge slvt she is...she just fvcks whoever is there when she is drunk...I'm scared to put my d!ck in her any more, condom or not. So my policy has a lot to do with the woman.