I was not tolerating his clear lack of respect, but any correction I had to make was so minor on my part with major exaggerated responses from him.
Your problem would have been sorted a lot faster if you had not given 'minor' corrections but made it one major one whereby you put the fear of humans into him and chased him so he was moving backwards around the stall until he was showing signs of submission. You make him think he is going to die.
As for him mounting the mares, it is not so uncommon, I have had flighty mares that have seduced a quiet gelding into covering them!
I would insist that he is only turned out with geldings as the covering can cause mares to become infected with germs.