Yes, I sometimes (too often) search the internet for “spandex butt shots” and came across this one:
(click on the thumbnail to see the photo at the source). Note: I was also interested in this photo as it demonstrated something I’ve been unable to do since tearing the meniscus in my knee.
I took a look at that person’s photo stream (Ida Pyl) and notice that there were lots of butt shots, but no photos showed a face. Something didn’t look quite right; then I saw the tags: Ida is a cross dresser, and, well, straight.
Ida’s whole photo set is some sort of mixture of butt-visible pantie line fantasy, “bondage” (Dr. Frankenfurter) and some good art.
Surprisingly, I found myself LIKING it!!!!
So, I started to question my own sexuality. Sure, I’ve never been attracted to a male in person, and being in a male locker room causes me no discomfort at all.
It turns out that I was reacting to the feminine or the “feminine looking”; example: (not Ida):

(click on the thumbnail for a larger shot at the source)
So what about the crack in my title?
Well, what I’ve found that these crossdressers WANT you to look at their photos and butts; some will even ask you for suggestions as to the kinds of bends, butt positions, clothes, etc. They are happy if you are happy!
With humans with two X chromosomes: things are a bit too complicated. One one hand, they get mad when you look…and then get mad when you don’t? If you told a woman “why don’t you bend over this way” or “go into yoga child because I like the way your butt looks when you are in that position”, they get upset.
Cross dresser: they say “thanks for the suggestion” and “hope you like it”:

Yeah, I know; the cross dressers can move to a fantasy world, then come back out of it whereas a woman is a woman 24 hours a day; she might have legitimate fears of unwelcome advances (or worse), being judged for her appearance (say, on the job), etc. And given that women are so much judged by their appearances, their “stock” goes down with age.
Sure, I am in my early 50′s and yes, my body is falling apart too; I can’t run as fast as I once could nor can I lift nearly as much weight. But I am employed at a reasonably well paying job and that counts for a great deal, even if I don’t look so hot.
Oh well, life has its complications and human sexuality can be both simple (“hey, nice butt!”) and complicated at the same time.