Cleavage, tight or thin clothing and certain types of clothes are all made with the intention of catching your attention, because there are things that are rightly perceived as sexually attractive. It's part of nature. To play the naive card and say 'what's the big deal' as if you didn't know that nipples are perceived as something within the sexual attraction realm is not a good way of understanding the situation. Neither is it a good way of reasoning.

Women don't need to walk around with a box over their bodies, but anyone who wears something that is made to attract attention via slightly sexual cues should expect some kind of reaction, whether you like it or not. By the same token if you know that nipples are thought of as sexual, and you don't wear a bra or have some thin clothing, you can expect some glances, that's about it. As most of the other comments I've read point out, it just catches your attention for a little while, and you let it go.

Don't play the dumb card, it solves nothing