Cis men will disguise themselves as women (rather than as cleaners, maintenance men, or just waiting for a quiet moment), to install secret genitalia cams does sound a teensy bit paranoid to my admittedly male ears...
Here's how I think of it. This seems less paranoid to me:
We know some small but significant percentage of men will abuse easy access to women's spaces to harass and assault the women there. We agree that women should be reasonably free from worry about such men, in designated women's spaces. There are currently a number of legal and social barriers that prevent such men from gaining easy access to women's spaces. These barriers make it difficult for such men to gain access, and make such spaces reasonably free from worry about such men.
Should we lower the barriers to easy access for men into women's spaces? The argument in favor is that it's not fair to transwomen to make them carry papers and respond to interrogation about their sexuality, just to gain access to women's spaces. It's not respectful, and it's not tolerant. A transwoman shouldn't have to go through surgery and hormone treatment, and bring a note from their doctor, just to enter a women's locker room. They should be able to simply declare - as ciswomen do - "I'm a woman", and enter. In fact, even the declaration should be unnecessary. Ciswomen don't declare anything, they just enter. This is the ideal solution, for transwomen.
However, lowering the barriers against easy entry makes it easier - probably too easy - for that small but significant percentage of abusive men. Those who were previously deterred by the legal and social barriers no longer have that deterrent to keep them in check. Women who previously depended on those barriers no longer have them to depend on. Not only does such a policy make women's spaces less safe for all women, but it also makes those spaces substantially less free from worry, for the women who use them.
I would like to see a proponent of trans access to cis-gendfered spaces address this conundrum head-on. No quibbling about definitions. No repetition of the legal and social barriers in place today. No whining about transphobia. No dismissing the questioner as paranoid. Just answer the questions: Should we remove the legal and social barriers against men in women's spaces, in order to better welcome transwomen into ciswomen's spaces? Are you prepared to address the concerns and the trade-offs that would arise from such a policy?