I wonder why the state cares so much about penises. There's just about nothing that gets the state as riled up as allowing photons that bounce off your penis to enter the eyeballs of other people except in rare circumstances, such as in locker rooms and at the doctor's office. And if those photons enter the eyeball of a human not yet 18 years of age, you are a particular target of the state. Even photons from a two dimensional representation of a penis are not permitted to enter eyeballs most of the time. If the penis itself is attached to a juvenile human, woe unto those who behold it.
What's the big deal? Half of us have penises, and the other half surely know what a penis looks like. I don't reckon anyone was ever damaged by the sight of a penis. So what is the state's interest?
It's not just the sight of the penis that the state cares about. It cares about where you put it and who touches it. Why?
I suspect that it's sex. Penises are part of sex, and they must be mystified and subjected to policing. The state would like to control all aspects of sex. Nobody should experience pleasure that it is not either controlled by the state or that does not result in a benefit to the state.
To the extent that sex leads to reproduction, the state gets new subjects. Otherwise, it's just useless pleasure that people can, if the state isn't vigilant, enjoy without state intervention. In a perfect world, the only time anyone would experience sexual pleasure would be incidental to attempted reproduction at times and under circumstances dictated by the state. It may even be possible with technology to render it unpleasureable or even painful some day. In vitro fertilization may render it obsolete and pave the way for total prohibition. This is probably decades away, though.
Already, as far as the state is concerned, the only officially sanctioned sexual activity is vaginal intercourse by heterosexual couples who have been licensed by the state. Everything else is illicit. Within the permitted arrangements, the state can be more or less indifferent to whether the participants take pleasure in their state sanctioned sex because this pleasure will not last very long. It will fade, and the frequency of the act will diminish steadily until most couples are all but asexual. Penises will not be unduly visible even within the homes of married couples.