Through careful study of Why Women Hate Men (terrible title, entertaining blog), I've figured out why guys send hideous dick pictures to women. It's because when women say that they want cock, men are taking this literally, and offering proof that yes, they have a cock, and are thus 100% qualified to meet this need.
I also blame certain porn genres where women show up perfectly groomed and perfectly submissive and of a specific body type, and men... show up. Talk about low expectations. You just have to prove that you're a man, right?
While I'm blaming things, I blame the weird hyper-homophobia that makes guys go "well, I can't tell if that looks good or not, I don't like cocks," which, well--straight women don't go around in sack dresses with matted hair because "how am I supposed to know what looks good on a woman?"
And I blame ignorance about how photography works. I think some people think that a picture of an object is a picture of it, and that any photo of their cock is equally attractive because it's the same cock every time. The idea that staging and lighting can make two equally "honest" pictures give very different impressions seems to be a little too advanced for certain people.
Which leads to my final source of blame, which is plain old rampant stupidity. Seriously, dudes of the world, if someone wants to see your cock they'll ask. How about you wait for that and go from there.
(Now I feel weird that I've posted pictures of my, um, lady-cock. But I do warn what they are rather than mailing them around to anybody who contacts me, and I do try to make them non-hideous photos.)
(Now I feel like "lady-cock" is either the best or the worst female-genitals euphemism ever.)