Others may be more important, or starker, or more common, but the gendered behavior I find most puzzling is the unsolicited genitalia picture.
Seriously. There is a (small, but evident) group of men who think the single most attractive thing they can do is send a woman they barely know a picture of their erect penis, to the point that nearly all my female and female-assigned friends who do online dating have received a dick pic. (Although I do not know any men who have sex with men well enough to inquire of them, commenters are encouraged to share their experiences about whether this is strictly a straightboy thing.)
Why? Why? WHY DO PEOPLE DO THIS? I mean, most men who like women are not attracted to a random disembodied vulva floating in space; why would they assume women who like men are attracted to a random disembodied cock? (Of course, there are people who are attracted to random disembodied vulvas/cocks, but I’m fairly comfortable with saying they’re in the strict minority.)
Side note to those who are considering sending a dick pic to a woman: please do not send one unless it is specifically requested, and I mean “I would like a picture of your penis” specifically requested. If she just asks for a sexy or naked picture, then take a picture of your torso, ass, or full body. Your dick can certainly be in the picture, but it should not be the starring role (barring being able to suck your own cock, or other exceptional traits).
For everyone else: I will now speculate about why people do this. If you have personal experience, please do elaborate in the comments.
1) Penises stick out, and therefore are easier to take pictures of than vulvas; if you could take a picture of your vulva without contorting into some completely awful position, then a whole bunch of women would be sending allegedly sexy but actually gynecological vulva pictures.
2) A lot of people believe that penises are the male equivalent of breasts. (This showed up in the comments on my post last week about why women should be allowed to take their shirts off in public– “I don’t want to see penises waving about in public, so it makes sense we’re not allowed to wave breasts about in public!”) Therefore, just like men are famously attracted to breasts, women are attracted to penises! Except that the male equivalent to a female chest is, you know, a male chest.
3) A lot of men are under the impression that male chests and asses are not a sexual thing to women, possibly related to the widespread cultural insistence that the only physical trait that matters is a large penis. (This has been changing recently to the insistence that there is such a thing as male beauty and it is defined as HUGE MUSCLES, but the first idea still exists.) I have no idea how they’ve managed to keep this idea. Do I need to show you people more pictures of hot shirtless men? Because I will.
4) The idea that every woman is incredibly attracted to a large penis, and that by taking a picture of your cock you’re showing that you have a large penis, which is hence very attractive.
5) The theory espoused in this Reproductive Health Reality Check article, i.e. gendered responses to insecurity (such as the insecurity involved in hitting on someone via the Internet). Men tend to respond to insecurity through asserting their physical superiority (“look at me, I’ve got a giant cock!”) whereas women tend to respond through seeking reassurance.
6) Men are incredibly more likely to hit on women on online dating sites than women are to hit on men. In particular, terrible men make up a substantial percentage of any woman’s inbox on online dating sites, as they will copy-paste the same “hey sexxxxxxxy ur cute wanna fuk?” message to two hundred women, while the decent people will send a thoughtful message to people they like. It only takes a few terrible men sending pictures of their penises to hundreds of women to make sure everyone online gets one.
7) Emotional armor. Hitting on someone makes you incredibly vulnerable– just think about the fear of rejection!– and the male gender role is about not being vulnerable. For some reason, a penis picture may make them feel less vulnerable. (If nothing else, they are literally showing that they have the balls to do it.)
Anyway, that’s all the theories I have. Add your own in the comments.