Adzeth: ...and I know this might sound crazy, but a lady wearing revealing clothing might be wearing 'em without intending to wear clothing that is revealing. Like some crazy vegetarians wear hunting clothes because they're amazing and appropriate for the weather or stuff. :p
It's certainly possible in certain situations, but I think overall if a girl is wearing sausage-casing jeans it's to make her ass look good. If she's revealing stuff in a classroom or at a club it's surely to attract attention.
I think more than the heat of summer you might say they are motivated by being "in style." For example if its hot you want to wear shorts, but why do they have to be short shorts that go right up to your ass? Probably because that's what's cool right now, what's "hip" as the kids say. So you might not be motivated by exhibitionism necessarily, but by conformity.
Even then though, the fact you chose to conform to a sexy archetype means you can't complain when people look at you as sex object (within reason). Same thing for people who do sex acts for attention or acceptance and then complain when they are treated like sex objects, you put yourself there.
Really it all comes down to social mechanics and weak people desperate for attention and acceptance, but then they don't like some of the attention they receive because of perceived value principles from competing influences (i.e. parents, moral leaders).