This argument, that a man who wants to assault a women won't be stopped by a sign - profoundly misunderstands how crime happens.
Men don't wake up in the morning and decide "today's the day I'm going to find a woman to assault."
Crime happens when people are enabled - when they feel comfortable doing it - and when they think they might get away with it. If you make men feel comfortable in a women's facility, by believing, acting, and encouraging them to be there, saying they belong, then it can tip anyone on the verge of committing a crime into doing so. And it can make the crime that they commit, worse. Instead of staying outside, they go inside. Instead of just staring, they touch. Instead of touching they, use force, and so on.
Well-lit carparks: Nobody who has decided to break into a car is going to be put off by good lighting: it just helps them pick the car more easily!
Cash-boxes with locks: Nobody who has decided to steal money is going to be put off by a cheap lock on a cash box - they'll just steal the box!
Assaults on staff: Nobody who wants to assault a staff member is going to be put off by a little sign!
There are a million ways to reduce - not prevent entirely - but reduce crime. And one way is to make the environment uncomfortable for potential criminals. The more uncomfortable they feel - the less welcome - the more likely they are to turn around and go home.
If you want to encourange assaults on women by men in women's facilities - just make men feel comfortable there.