If you don't want their rules there's always another site that you can visit and spend your Bitcoins. They have their own set of rules and we have to respect that. We cant just register, sign-in and then when we found a rule that is not suited for our taste we file a complain, it's not like that. It's their house, they have the right to say who can come in and where do you sit and where to do eat, if you don't like their house rules, go out and find another one.
There are things that are acceptable and things that aren't. You're correct, it's their hous and they make the rules, but only to a certain degree.
There are aswell laws against gender based discrimination (/gender based discrimination) and those are standing above the house rights.
A brand managed by an US based incorperation (Las Vegas, Nevada) should be following those laws against gender based disctimination, and not violate them.