Seriously. People spend thousands to travel to Pompeii and walk through tittering about how naughty the Romans were. It's not as if those Romans were any different from the guy drawing dicks on a blank webpage. And if 2 measly millennia is all it takes to confer archaeological significance on phallic graffiti, why not just get ahead of the herd, take the long view (so to speak), and appreciate your own culture in all its messy future significance in the here and now?

There's nothing wrong with it morally, at least in my opinion.

There's just the practical issue of how to deal with all the penis drawings.

Also, it is just annoying behavior. People think they are being funny, but it is the equivalent of a child writing P O O P everywhere. It is mildly fun for them, but tiresome for everyone else. It isn't in the spirit of the group effort, and just takes the fun out of it for people who actually want to make something.

I guess penises do not belong to the movie being drawn.

Well, the problem is some of those aren't even drawings. Some minimum effort should be made even if you don't want to trace the frame.

by allowing penis drawings it pulls any artwork generated to be penis focused. Crowdsourced art suffers if it can only ever be about the penis is mightier than the sword.

Bad drawings likewise limit the aesthetic of what is possible.