makes the implication that only lesbians wouldn't welcome a penis bearer in a same sex spa dressing room.

That doesn't really hold up here because this exact situation is about this presumably straight woman, and a bunch of other presumably straight women not wanting to see a man in a women only space.

We aren't inside her head so we can only make assumptions but her Instagram is very right wing and religious. She likely understands that a man in a women's space is dangerous. She likely realizes that women don't typically enjoy seeing random naked men at the spa in the women's section, regardless of sexuality. She also likely understands how many straight men wouldn't mind seeing a naked woman at the spa (I'm reminded of straight men who say they would love to be hit on and catcalled whenever women describe how uncomfortable it makes us). So she probably realizes that turning a mirror to what she experienced wouldn't make that guy understand. The best analogy would be to say 'well you might not like women.'

That doesn't imply that only homosexuals would bristle at a situation like this. It's acknowledging the differences between male and female socialization, and working around the reality that many men like him don't understand why single sex spaces need protecting.