I see it at work mostly with leggings-as-pants because ones that aren't sheer when standing can become sheer when getting something off of a bottom shelf. Pencil skirts are classic for butt cleavage as are trousers. Loose flown skirts get caught in butt cracks all the time. Also, reaching for the bottom shelf item while in a knee length skirt still tends to show the thong and whole butt when bending instead of squatting. The guys like it though. They also like summer because the braless women come into the air conditioned store and then all the ping-ping starts.
I've come to the conclusion that if random casual acquaintance dudes really get excited about your outfit, they're seeing something that they think is illicit.