mucous membrane

I need to better understand how condoms can protect me from STIs.

fox
asks:
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....