Maybe friends or others told them they had a nice ass. Maybe they liked the ego boost.
Maybe they know that on this site pics of their asses will get appreciative comments and they will get off on that. Actually I kinda fall into this category, as there are pics of my ass in my gallery, and I would never open it to a guy's cock. It's just for fun.
Also, there are guys whose gf finger or dildo, and they like it because it's a feeling that has no sexual orientation. The area around your asshole is erogenous, and tickling the prostate with a dildo or a dick is said to be awesome. Gay people experience that, but actually why should this source of pleasure be banned for all other guys ?
I don't think that on this site labels are to be viewed as limitations. I see many posts where straight guys are constantly questionning other straight guys as to what they show or what they say.
There are many interpretations of the labels, some are more lax towards experiencing or plain fun without any emotional aspect.
I think I'm straight because I never had and think I can never have, any emotional connection with a guy, go out with him, live with him or whatever. I fool around with guys for fun. Many here would disagree, based on their own definition.
But it's my life and I hurt no one by thinking and acting so.
So why would straight guys bend their asses on their pics?
For fun.
Because they have no complexes.