I'm unclear on how condoms are supposed to be effective in preventing female-to-male contamination during "plain" sex, I mean insertion of the penis into the vagina. Let me explain.

Latex is an effective barrier to virii and germs. I get that. As far as protecting the woman is concerned, I've no trouble believing it works. The STD virii or germs are present in the semen and/or pre-cum; these are "emprisoned" by the condom, don't get out, and don't get into contact with any part of the anatomy of the woman. She's protected. The sweat of the man does not contain these virii or germs and thus no risk with the rest of the skin-to-skin contact. But in the other direction, I don't quite get it.

Hi, I'm sixteen and about four months ago I was treated at the hospital for severe anemia due to over excessive menstruation. While I was there, I had to have a pelvic exam done, and I'm already really shy, and I've never been touched like that or even have had a boyfriend. So the doctor (who was a man) was about to do it, but I was so scared he had to physically spread my legs apart. Then he put the speculum in and did whatever, but he had to push through the hymen, and it hurt pretty badly. It seemed like he didn't care at all how I was doing, or anything. Now I cringe when people mention sex or anything like it because it reminds me of him and the pain and embarrassment. How am I ever going to trust a man enough again to let him get close, and how can I block this event out of my head?