Please do. If you consider a company expecting employees to have the maturity above an adolescent boy "bad employers", we'd like you to help filter yourself out.
Don't worry, you probably already filtered me out. I have plenty of ponies shown publicly in many places, including github profile. That degrades my maturity level to a little girl - and I'm not even a woman! [/s]
Any company judging "maturity" of actions done by someone in their spare time is not worth working for. It's just not their business to decide whether someone is "mature" or not, so if they cannot manage their business properly, why should I want to work with them? They probably won't be able to maintain healthy relationship with their employees anyway if they're so keen to filter out perfectly fine candidates over something so unimportant and nonharmful as low-grade sense of humor.
Liking something childish and acting unprofessionally are very different aspects of immaturity. I wouldn't care about the former, but the latter may reflect poorly on the company, especially in open source where developers are like ambassadors to the community.
But if you maintain a good wall between your professional life and that immature spare time, such that the employer doesn't even see the low-grade stuff, then fine.
How does employer seeing it or not make any difference? (except the obvious one that it's hard to react to what you can't see)
It's hardly even "acting unprofessionally". It's a lighthearted project where, well, some part of the fun is waiting for the first person to draw some dicks in. If you can't get over it, that's more likely problem with your attitude, not the person's who drew it.
If the employer is worried about how you represent their public image, and you've disassociated this activity well enough that the employer doesn't know about it, then their concern is mostly satisfied.
Anyway, let the mutual filtering continue. You don't want to work for a company that cares about drawing public dicks, and such a company won't want to hire you. It doesn't really even matter to decide whose attitude has a problem -- you're just culturally incompatible.
