This is such a dumb argument. There are plenty of similar crimes that are difficult to catch in the moment but are still illegal. It is difficult to prove someone has shoplifted unless you confront them in the act and check their person. That doesn't mean we make shoplifting legal (interesting side note: California actually did make shoplifting somewhat legal for a time and it led to absolute chaos, which is why we don't just legalize criminal behavior even though it is difficult to police).
Being illegal and having criminal consequences is enough of a deterrent to stop most people who don't want to deal with the consequences of committing a petty crime, greatly reducing the behavior and victimhood. For the rest, they can be confronted by people who feel comfortable calling them out, bystanders can alert nearby police, they can be caught on video entering or leaving the facility, and once the ridiculous law allowing sex markers on ID is overturned (the natural next step after the Supreme Court ruling) checking ID will be all the proof anyone needs.
But like most crimes, it's pretty impossible to 100% stop the behavior. But that doesn't mean we legalize them. Can you imagine! And frankly I hope this makes you ask yourself, if you know the majority of women don't want you in their space and consider it a violation of their personhood, if you know it is illegal, if you know you can get arrested and face criminal consequences, and you are still so compelled to enter a women's bathroom that you go in anyway, I think you need to ask yourself what is it really that is compelling you to go in there... Bc I assure you its not just bc you need to go to the bathroom.