Search Results: guys showing their butt

Just to get this straight: even though I wouldn't be completely disgusted or personally offended by it, I too find it weird if I can see someone's thong or underwear somewhere in public; bet it because they stick out or because their yoga pants are see-through. But what is this inappropriateness about? Is that that we can see other people's underwear lines or perhaps even more than just the "lines"? Is it that if we can see their thong it's very likely the case we can also see their butts (at least when they wear see-though clothes? And which one "weighs heavier" in your opinion?

This question might seem random, but I was out today and noticed two girls' thongs that were clearly visible. So my first thought was like "Don't they know better?" and "Why do some people not know how to dress appropriately, wear belts, etc?". And after that I wondered (kind of on a meta-level) what it is that we find so inappropriate about this. At the end of the day, it's just underwear, no? So theoretically, you could say "what's the big deal?".

Hence my question, and please be honest: what's the big deal? The underwear showing? The "nudity" that might be involved? Or something else?