Yes that makes no sense. I think the whole thing is about upbringing, though. There were now-well-known cultures way back when that weren't concerned with the covering up of genitalia, but then all of a sudden it was a drastic social offence to even show some leg skin! Nowadays, it's going back to how it was before, with girls wearing things that make you wonder why they even bother putting anything over their underwear at all, but I digress.
I was raised on the idea that crotches, butts and boobs must be covered at all times unless at the beach (that last part never took hold for me though), so I feel that seeing the fleshy part of a boob is just as "bad" as seeing the nipple. Jeez this feels weird to type. xD But maybe other people are similar. I assume you post these things to start discussions? Anyway, more recently (a lot due to pop culture), being naked has become more "acceptable", and the importance of covering up so-called "private parts" has become gradually less important. Personally, though, I thought Miley Cyrus' only-nipple-covering outfit (I don't remember when from) was appalling in class and design because you could still see what was essentially her whole chest, even though it was technically not public indecency punishable by law.
It mildly comes to my attention that my entire rant was on something different to your original question, but hopefully there is some interconnected material within. o3o;
Regarding males' vs females' chests, it seems to me that anything besides the face that excretes a bodily fluid is covered up. I can't say why though. I mean underwear makes sense and all for support and such, but for the people who don't wear it, ... the reason the majority of society feel the need to believe that certain parts of the body need to always be covered up is something I don't actually understand.. o3o; Of course I am such a member of society, but I can't identify any underlying reason besides "that's how it is".
Oh, but studies have shown that wearing clothing and in particular, covering up the "sensual" parts of the body, encourages people to be more selective about their partners due to the fact that excessive production of hormones that push someone towards prematurely procreating is less frequently caused. [Source: Vsauce - Why Do We Wear Clothes?]
Maybe nipples were just really attractive to the people who made the rules that nipples should be covered? But just because clothing regulations may not have been created with a proper reason in mind (though I'm certain it's quite likely that there is one), doesn't necessarily mean that clothes are useless and should be abandoned entirely. For one thing, they're warm! :3
I'm pretty sure I've almost entirely missed the point of your question, but nyeh. You can question me if you want more spam! X3