To know so little about how women operate in the world and what is a threat to them is worrying. You might think that the poor dears are fraidy-cats and not willing to have a laff, but men don't grow up in a culture of constant threat. Count how many girls and women have been raped and killed this year alone, compared to boys.
You seem to be under the impression that the rape of men isn't an issue (in terms of numbers). I'm assuming you meant men rather than boys specifically?
Taking the first hit in google (for the UK), 69,000 women and 9,000 men were raped a few years ago, _BUT_ there's a fairly pervasive theory (advanced by a woman, if that matters to you) that only in 1 in 10 male-on-male rapes are reported due to the social stigma.
Female on male rape, historically, hasn't been treated with anywhere near the same severity as male-on-female, so a reasonable number of victims either don't report, or get nowhere when they do report.
If you want to try and correct 'sexist' views, feel free, but making generalisations about rape rates simply makes you look an idiot - rape is an incredibly abusive act, whoever the victim, and comparing numbers does nothing but further dehumanize the victims.
There is no such thing as "it's just a cock pic" if the man and the woman are not in a relationship. It is always a threat. It doesn't have to mean that an attack is about to come, it just says, "I can rape you."
I think you're unfairly generalising here too. I know and have known women who would probably laugh this off just as some have been doing in the comments. I've also known blokes who would have been incredibly put out by receiving an unsolicited picture like this (whether cock or tits). People differ, simple as.
Not, mind you, that I'm saying it's ever appropriate to send something like that to someone you don't know.
The men who do not get this are seen by pretty much every woman as one of the men who will sens cock pics to women he doesn't know.
For the record, I disagree with most of the generalisations you've cast, but I'm definitely not one of the ones who'd think it was OK to send pics like that.