Sure, there's lots of gear that can be worn by both genders. What relevance could that possibly have? The point is that there is also other gear that cannot, and that's where the inequality lies.
No more so than seeing females in it would scar the rest of the audience.
Twenty to thirty years ago that statement would have actually been true. These days it's not. Players are about 50/50 between men and women. (The action, first-person shooter, and sports game genres do still have a significant male majority, but RPGs don't.)
Though not as relevant to the dresses that have also been brought up, in the case of the bunny outfits, it's because the design of the outfit is supposed to look sexy. You can't accomplish that if you have to switch genders. Most people can only find one gender sexy, so if a "sexy" outfit is released, but not on the gender that's capable of actually looking sexy, it's going to frustrate people.
To be fair, I didn't see a single post in that thread that said only the Setzer outfit should be made unisex and not the bunny outfit. There was more focus on the Setzer outfit because it's the nice one, but pretty much everyone who mentioned both seemed to think they should both be unisex or have equivalent versions for both sexes.
In the case of the female only dresses, no, which is why those don't really bother me. In fact, making them available to both genders wouldn't in fact be all that equivalent, because they'd have a very different effect on each. Females would wear the dress to look pretty, while males would wear it to look funny.
In the case of the bunny outfit, though, I do think it would get worn as much by males as females, because it's the same effect for both. Chippendale bunny boys are every bit as appropriate as Playboy bunny girls. (There could be some doubt whether either one is appropriate, but if SE has already decided to introduce the one, then they should be willing to introduce the other as well.)