A good deal brighter than you, it would seem.
Let me explain it in really simple terms.
The law says that when a space is reserved for one sex, as is the case with eg toilets, changing rooms, prisons, then the other sex cannot be admitted. There are two reasons for this. One is to keep women and girls safe from men. The other is to maintain privacy and dignity for each sex, but particularly for women.
Very occasionally, women might breach this rule by using a men's toilet (guess what, women aren't choosing to go into men's prisons or changing rooms). Why? Because society has so little regard for the needs of women that historically it hasn't provided adequate toilet facilities for them. Women, as we all know, take longer in the toilet than men, because they need to use cubicles rather than urinals and because they sometimes have periods to deal with. Quite often, too, women have more urgency than men because of pregnancy or because of bladder problems as a result of childbirth. That means that very occasionally, women will use the men's toilets – not because they want to (I never WANT to use the men's toilets) but out of sheer desperation.
This, of course, is very different from the reasons men use women's toilets. And in a sensible, tolerant society, people will accept the reasons a woman might, now and again, use a men's toilet.
It is possible that you are extremely stupid and don't understand this. I think it's more likely, however, that you understand it perfectly well and are pretending not to understand it. It makes reasoned debate on the issue almost impossible. Is that your intention? Or do you just enjoy looking like a fool?