I don't discount the possibility that there are dumbshits who leave it up to chance - but you can usually spot those from very very far away.
Leaving the unusual circumstances aside (like genital bleeding, open sores, etc) the rate of F2M infection during vaginal sex is abysmally low. For a male to get HIV from a female from vaginal sex is almost unheard of according to the study I read about 5 years ago.They do a pretty good job preventing HIV, which is the scariest of them.
Now, its entirely possible the study was conducted erroneously, but I can't look up the stats right now easily from work. If you have evidence to the contrary, I am open to reviewing it.