But, as you might have guessed, I am not writing this just to remind your of your body proportions. No, there is something more important going on, and the attempt to hide your ass is just the symptom. By hiding your ass, you are either expressing insecurity about your body, or else you are showing your lack of comfort with male sexuality.
Granted, the guy who checks you out from behind probably isn't being completely honest either. If he were, he wouldn't wait until you passed to "steal" a look at your ass; he would do it while you are watching. Of course there is the practical consideration that a girl's ass can only be seen from behind, but this doesn't change the fact that most men are too scared or ashamed to express their sexual interest in a girl directly. Those guys probably liked your face or chest more than your ass, but checking those out would have required eye contact and revealing that they like you, which of course requires facing rejection. Anyway, just because most men are insincere in looking doesn't excuse you for being insincere in hiding. I am only saying so to point out that this isn't a gender-specific problem.
You might attempt to defend this uneasiness by saying that you "don't like being treated like a piece of meat," but this is just an example of clever semantics putting a negative spin on a phenomenon that is so natural that you couldn't - or more accurately, wouldn't - exist without it. Straight men who are unmotivated by an attractive female body are about as common as women who are unmotivated by the idea of commitment. Good luck finding them. You either need to get used to that idea, or get used to the idea of being single, because if you can't feel comfortable with the fact that a man feels sexually attracted to the curves of your body, you can't feel comfortable with men.
Let me be clear here: I am not trying to defend men who only care about a woman's looks (who, by the way don't exist - but this is the topic for another post), and I am not trying to defend rude or crass expressions of male physical desire, like cat-calling or groping. But I am trying to point out that it isn't normal or healthy for women to walk around constantly worried about whether or not men are looking at their asses. And it certainly isn't normal to awkwardly pretend that your limbs happen to be in conveniently "concealing" locations when they have no natural business being there.
So if you are one of the girls doing this, realize that men know what you are trying to do and it doesn't work. And if you want to dig a little deeper, recognize it as a sign of a psychological kink that you need to work on.