They feel some pride that their penis generated this strong reaction. But if we remove as much of the reinforcement, including negative attention, many of these men might view this behavior as ultimately unrewarding.
So, shaming, banning, flaming strategies are unlikely to be successful. It's important that women, and men, understand that this behavior, even when it feels distasteful and rude, is not personal.
Many women are genuinely bothered by receiving such pictures and view it as an unwelcome intrusion.
Some feminist writers have described this as a form of sexual assault, and a way in which men assert the dominance of their sexuality over that of women.
This definitely appears to be more of a male thing.
Ultimately, the answer here lies in greater dialogue with men and women, over what they actually want in sexual communications.
This reach some of these men, those who are genuinely not understanding some sexual differences.
That distinction may simply be an expression of gender differences in mating and dating strategies.
The fact is, women are told that being sexually bold in such a manner is shameful and makes them a slut.