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.