It's a cultural thing. If you were to "hang out" with, say, a tribe in New Zealand whose members were comfortable with nudity, it wouldn't be wrong. But it *is* wrong to expose yourself to anyone who would be made uncomfortable by it. So why are people uncomfortable with it here in America?
It's partly because we live in a temperate (fairly cold climate), where clothes became the norm. And we're not used to seeing "private parts" outside of a sexual context. Therefore, sudden nudity can make a platonic context seem sexual.....and that (understandably) creeps many people out.
I don't hate my body. In fact, I think it's pretty slammin'. I don't hate the bodies of some of my male friends. I think they look pretty slammin'. But I don't wanna see their equipment. A dick can be a dangerous thing and even if I'm friends with the guy, I don't wanna be looking at him and thinking, "Hmm. Better keep that away from the food. Is it getting stiffer? Does he expect me to act impressed? Will be resent me if I don't? I just want outta here!"
Likewise, if I was stark naked in public, I know I'd get more of those "I wish I were screwing her right now" looks from guys, which also happen when I'm fully clothed. Those stares tend to make me think, "Back off! You don't know me! You don't appreciate the person I am! This isn't for you---it's for someone whose earned the priviledge." I'd feel like the guy was stealing from me somehow.
Of course I want to be thought of as attractive. But I'd rather a guy's thoughts be more like, "Wow, she's pretty. If she's as good on the inside as the outside, I could see myself with a girl like that."
But if a guy has no choice but to look at a my T&A, his mind is more likely to forget about the person I really am and he'll go straight into "I wanna have sex" mode. And that just makes me feel undervalued...and possibly nervous...y'know?