Even though they are often sexualized by others, breasts aren't innately sexual. The penis decidedly is. And as much as I love my fiancee, along with all his bits, as an isolated entity the penis isn't an attractive organ to look at. It's a wrinkled and oft discolored, drooping piece of wrinkled smelly flesh. Moreover, many of them are attached to men that most women have absolutely no interest in. Even if there were no moral or ethical argument to be made (e.g., indecent exposure to minors), from a purely aesthetic perspective this has pretty much zero chance of catching on.