that would mean that lesbian brothels are actually the meta 😳Anti-discrimination laws (and rules of thumb) can not cover sexual preferences or opinions on aesthetics or such. Requiring a brothel (or a specific sex worker) to go both ways is as absurd as requiring everyone to be bisexual.
Also, it being women-only probably helps with its legality if laws define illegal prostitution as something involving penetration - a claim about Japanese laws I've seen in other h-manga (idk if true or not).