Why do some of us men actually think that showing their genitalia to a (usually)relatively less ‘powerful’ woman increases their sexual desirability? From an anthropological and animal kingdom perspective, is it somewhat equivalent to males displaying the size of their horns? Perhaps it’s no coincidence that the men who do engage in such behaviour are also usually people who are considered ‘powerful’ in society and therefore ‘better able’ to sire and protect their progeny.
I don't think it's about increasing their sexual desirability though. It's more about power, control and attention.
In the animal kingdom, a stag showing the size of his horns and his fighting prowess might well be a big attractant to a female, but in that world the female is looking for and expecting such behaviour. It's part of their mating ritual.
In our world, getting your knob out in front of a female on a train or in the office is completely different. It's unexpected, unwelcome, and breaks the rules of social acceptability. Therefore, it's an aggressive act. In such circumstances, no female is going to be impressed and think "ooh, I'll have some of that."
The feelings of the victim have been completely disregarded for the gratification of the aggressor. Therefore, it's insulting and often frightening. That is not attractive, but it's not about that anyway.
Some men get off on female discomfort. It brings a sense of power. In that moment, they are getting a reaction. In that moment, they matter and are having an effect on a woman/girl. That can then arouse them because they get off on basically making a female squirm. It's about having power over someone.
Many who indulge in such behaviour are not powerful in daily life at all. Many are rather insignificant. Exposing themselves to unsuspecting females can alter that insignificance in their minds, at least momentarily. They go from basically being ignored by females to grabbing female attention by the throat simply by getting their dick out. That's not about trying to attract a female. It's about getting attention and having control over a female in a situation you have instigated. So, self gratification at the expense of their victim.
The rich and powerful who indulge in it might have slightly different motives mixed in. For some, it may be about entitlement. Some just think that money and power exempts them from the acceptable social norms and boundaries others have to follow. But again, that's wrapped up in power and the control money/celebrity often brings.
Some who do this kind of thing will be highly insignificant males who otherwise feel powerless and inept with women. Some will be rich, powerful males who feel entitled. But whichever side of that fence a person is on, flashing their bits to unsuspecting females is not about boosting ones attractiveness. It's more about asserting themselves, having control and getting a reaction.