You're straight. I have plenty of guyfriends who comment on guys' butts. I think it's a little weird, but I think it's more of a comparison thing (noticing and admiring and appreciating as in 'I want a butt like that') than thinking of sex.
I have plenty of guy friends who notice guys' butts too and they are straight. My boyfriend actually points out other guy's butt, abs, pecs etc. He notices them because he'll point out how one guy looks & is like, "I want to look like that!".
I notice girl's bodies, but I'm not a lesbian. Sometimes, a body part can be interesting in the same way a sunset or a piece of art can be. If there is no sexual arousal, then its just an odd fascination.