I guess it comes down to whether or not the women in question actually requested the photographs of Weiner's, erm, weiner. If so, then he's just a regular sicko, which I guess isn't actually hurting anything. If not, then it's at the very least sexual harassment, which if I'm not mistaken is criminal.
The thing is, this runs into the same kinds of problems as determining if consent was given, in order to figure out if a sex act was rape or not. At least one of the women was openly talking about sex with him (quoted on Jon Stewart) and if you're already openly discussing sex, or even sending each other suggestive remarks, then abruptly sending dick pix is no more criminal than if you're flirting with a women in a private area and suddenly dropped your pants. Smooth? Hell no. Likely to get you laid? Of course not. But criminal? I can hardly see how, because then where do you draw the line between that and when you get naked in order to have sex?
I'm not sure what, exactly, indecent exposure consists of, but I'd assume it's something to do with PUBLICLY displaying your junk. In a situation where you're alone with another adult (which is sort of analogous to internet contact) and already having suggestive conversation, I can't imagine that suddenly showing up naked would be considered illegal. If so, then the law needs to change. Otherwise, a woman who invites a man to her place from the bar, waits for him to go to the bathroom, and when he comes back is laying naked waiting for him, could be just as well prosecuted.
If the standard for whether it's sexual harrassment is that the recipient has to have formally said "please send me pictures of your gentials," well....that's just like saying that sexual consent can only come in the form of saying "I consent to sexual intercourse with you."