As leader of a clan having a penis as a clan emblem shows you are pretty immature or not taking your clan serious.
... Hi, welcome to the internet, you must be new here
![Wink ;)](https://gunsoficarus.com/community/forum/Smileys/aaron/wink.gif)
Anyways, the only possible way to make user uploaded emblems a possibility would be if Muse had the staff available to go through and approve every one of them personally to curate that which does not belong, and I believe they just don't have the manpower to do that.
Edit: I should add here that I'm not opposed to your idea, I think it'd be awesome, just looking at it from a logistical perspective.