Hey dude,
You've got to accept a certain element of risk in your life as tough as that may seem. Either you accept it and live a happy life or you run for safety and be miserable. Do what is reasonable to protect yourself and from there live your life and handle whatever challenges come up with confidence.
With that said, it is unreasonable to demand a chick get tested simply because you have only slept with virgins. Sleeping with virgins is no guarantee that (a) they told you the truth that they are virgins or (b) that they are completely STD free (they could have gotten herpes from a toilet seat and the HPV virus could be present on her cervix due to some hereditary factors (Hey I've heard if happens!).
So anyways, I'm not saying this to scare you, but you need to accept a reasonable amount of risk. You can' t run from it. If she gets tested and is STD free you'll still have to accept that she may cheat and catch something two minutes after testing all clear in which case you're back where you started.
And finally, some STDs don't even show up on tests. For example, women get PAP smears to test for "abnormal cells" which could be a precurser to cervical cancer and which could have been caused by many factors one major one being a strain of the HPV virus that does not cause warts. Now a chick can get tested and be STD free today and then 6 months from now she's got an abnormal pap smear caused by HPV and did not cheat on you. She's at risk for cervical cancer and you've got zero risk because that strain causes no damage to men (no warts), but you may infect other chicks down the road. It is estimated that something like 80% of people have been exposed to this strain and most don't know it, have no symptoms, and do not test positive for it in STD tests (you can't test it for men and if a woman is healthy it doesn't really do anything and is undetectable too).
So the point is, no matter how much paranoia you think you'll cure by demanding she get tested, there are so many variables that you can't control so you're better off taking reasonable precautions and living your life.
I suggest the following:
1. Use a condom with the chicks no matter what. All the time and with each chick.
2. Get tested for STDs once a year and suggest to the girl that she do the same. Tell her that it's part of your yearly check up and that it's a mature thing for both of you to do together. If she agrees and you trust her, than you can trust that she won't cheat and will let you know if there is some health concern you should be aware of. If she objects or resists the idea then you should decide whether you can handle being with a girl that you don't trust, and boot her if you can't.
But as a sexually active male, you'll be dealing with all kinds of crazy sh*t from women and this is just something else that you need to manage first with your own emotions (i.e. don't worry irrationally) and then with the chicks.
Good luck.
Cesare Cardinali