What PigletJohn said. Isis are quite bad enough, without quotes of dubious veracity.
They like to boast about their actions, so why are they denying this? And how the heck would they enforce it? Mosul has almost 2 million people and Isis have a (military) force of 10,000, according to them. It's not going to have cultural support because it's not practiced in that area of Iraq (I don't know if it happens in other areas) and I think they're going to find it difficult enough keeping and defending the area without spending time checking people's fannies for compliance.
I'm suspicious of some of the pics coming out of there but usually it's Isis boasting - they don't have any problems with claiming responsibility for all the most gruesome things possible, whether true or not, so why would they be all shy about this? It doesn't seem to make sense.