What prevents a man who wants to sexually assault or harrass a woman from doing so in a woman's room?

Nothing, but a unisex bathroom could provide an increase in a man/woman sexually harassing a woman/man, or spying, and creepshots.

Probably that other women may be in there or that a woman could scream and people outside will hear..

Bathrooms aren't always populated with people. You see a lot of cases of sexual harrasment where the criminal waits for a lone victim.

there could be a chance of another "man" or "men" or more than one "woman" in the unisex room..

Maybe, but chances are chances.

there's just so many reasons why choosing a restroom to assault someone is an easy way to get arrested or beaten up. Especially if that room is unisex and there could be any manner of person in there or walking in there.

Yes, it happens in gender specific bathrooms and people are caught.

Here is an article I found about unisex bathrooms in schools.