I talked about this in a twitter post today, but I'm finding myself jealous of women because they have more body parts that men (and women) are curious to see. A titty flash has a unique place in our culture of being a bit "naughty" without necessarily being sexual: it can simply be revealing.
There are sexual connotations, of course, often related to the free-spiritedness or relative sexual openness of the flasher, but unlike a male cock flash (or cock picture), the message isn't "here is what I fuck with", but instead "here's a peek!" I'm not sure there's anything equivalent for men, especially anything that doesn't scream "I'm showing off hoping to attract you as a mate!" - I'm thinking anything shirtless showing off muscles, or the like.
There's probably a lot of baggage here tied in with objectified female sexuality and repression, and there is certainly a double standard in most places regarding having to covering up. But from an exhibitionist's point of view, I'm at a loss for how (or what) to show off that doesn't jump immediately into "overtly sexual" territory. There's a visual progression of flirting that comes with clothing removal for women that just doesn't seem to have a male equivalent. It's weird, and I'm jealous - I just wanna show something off.