The way I see it is, guys generally are fine with any pic of women's assets, like lots of girls also send just completely default titty pics and guys eat it up, so they probably think it works in reverse too.

Also there's plenty of women out there who are more interested in other features rather than dicks, meanwhile every straight guy is gonna be fine with any piece of titty or ass a woman shows them pretty much. I've talked to multiple women who didn't care for seeing dick but they loved seeing cum, for example. I agree though, I never understood the point of sending random dick pics, I think guys who do that probably are doing it more for themselves as some kind of fetish rather than to impress women?

As for the Grindr thing, I think that's probably because the audience there is more into seeing dicks than the average woman 🤭 I agree, it is worthwhile to learn that but I understand it's not something that comes naturally to most of us. Before I started hanging out in pornspaces I really had no clue either what women want to see and stuff.

Dick portraiture must be a profession, or at the very least a hobby.

Because due to law of large numbers, it has worked for someone. The 99% who are disgusted by it are just a collateral damage. Of course it might have been a much more artistic picture of a dick, but that gets glossed over.

There's always the exhibition and degrading kink to consider as well. Flashers do exists and sending a random dick pick is rather easy. For the degrading kink, sending a random dick pick is sure way to get at least some insults or disgust effect.

There might be just ego or inflated self esteem in the play as well.

Though just for clarification, I'm not in any way defending it. I'm probably more baffled by most sexual behaviors than the average person. I have just been curious over the drivers of most sexual behaviors.