People if you're desperate for breasts, porn exists on the internet and you don't have to pay for it.
The way I see it is that an actress showing her breasts in a non-comedic movie is considered brave (while at the same time leaves her open to be sexualized by viewers and dudes on the internet obsessed with baps). Like for an actress to show her breasts, that's considered her revealing herself in the biggest way, truly committing herself to the role. I imagine that if There Will Be Blood had called for it, Daniel Day-Lewis would have shown his dikk. But he doesn't have to. A male actor doesn't have to go to nudity in order to be seen as giving a "brave and revealing" performance.
As a side note (because I'm going for a bread-and-circuses approach), Scarlett Johannsen offered to go topless for The Island but Michael Bay said no. You can react to that however you like.