Rightly or wrongly, our culture tends to teach males that females are sex objects first, and people a distant second or even third. Consequently a lot of guys will give you "the look", sizing you up. Are you attractive? Do you have a nice rack? Does she have a sweet rear end? How about her figure? Her face?
You're going to get these looks for the rest of your life. In one sense it's a compliment, because someone is seeing you as the woman you are. In another sense, it can feel infuriating because someone is seeing you as a sex object and wondering if they can bed you.
I work in a white collar professional environment and there's a subtle difference between many white collar males and blue collar males. More white collar males will look at you discretely, and in an "admiring" way while more blue collar men just ogle you flat out. A few guys I've chatted with have said that the thought process is about the same though, just how they go about it.
And in today's society, like it or not, we're going to be ogled, whether it's overt or not. So I guess as long as they aren't pawing us or trying to assault us, it's tolerable.
But welcome to womanhood.