The scars do sound like stretch marks to me. You don't have to be fat to get stretch marks, they just happen sometimes. And the thigh/genital area/love handles are really common places to get them.
Has anyone checked you for a yeast infection? Did the girls get yeast infections? Just a thought. Men can have them and pass them on, but they aren't as obvious in men so they are sometimes missed.
But use a condom. I'm not going to get preachy or anything, but if you think there is something wrong with you you have the responsibility to protect the girls you are with. It is possible to use condoms long term, some people use them 100% of the time for birth control and std protection. It is just part of being a healty adult.
See if you can get tested for HPV too, since there's no way of knowing when the girl got it, you'd never know if you have it or not.
If you have a chronic bacterial infection (like trich.), you will need to go back to the doctors after you are done with all your medication so they can retest you and give you different medication if the original stuff didn't work. You will need to take it in your hands to follow up and bug them until they figure it out.
And I really do think that printing out your post and bringing it to a doctor would be helpful, because the doctor would know your whole history and how frustrated you are, and the different things you've tried.
Good luck. It sounds like you are having a miserable time. Please, though, don't have unprotected sex until you get it figured out.