So let's put this in perspective...
She is having sex almost exclusively with a group of people that has the LOWEST incidence of STD's in general population and you are worried.
She is having sex almost exclusively with a group that gets regular weekly or bi-weekly tests when the "average" guy/girl around may only bet getting tested once or twice a year, if ever.
She is having sex almost exclusively with a group that does not agree to have sex with anyone else UNLESS that have a recent (i.e.: 1 to 7 days old) STD test.
Having been with retired and current porn stars both as escorts and SB's, I can tell you that there is no group that makes me feel more protected from STD's. And that includes an 18-year old who claims to be a virgin. The porn star will have documented, verifiable proof, as will I. The 18 year old may be (and probably is) untested and lying. I have more than once gently probed (no pun intended) the occasional 18 year old and found that their definition of "virgin" is pretty flexible and can mean "only given BJ's and HJ's" and also may mean only anal. Or even "just had sex one time with my HS boyfriend." And they never have a recent test and they are never on birth control.
If, at the bottom of all this, you just object to the idea of her embracing a dozen or so dicks a month, then think about that 23-year old college hottie SB who takes your sugar to pay for her weekend in Vegas with her hottie girlfriends and ends up going to the bathroom and taking 3 dicks (aka air tight) from the rappers, football players, and bartender/DJ's she meets in that $1500 bottle service booth at a high-end Vegas club. Feel any safer now?