When they disrespect women.
In junior high I'd hear boys (who had no idea what the hell they were talking about) describe their ideal - or current, but somehow always in Canada - girlfriend... it sounded like they thought they were ordering fast-food. ("36-24-36, bazooms like WOW. Hold the ketchup.") This apparently made them feel powerful, basically behaving as though they were so desirable that they could pick and choose the most minute details of whichever woman they would deign to cast their gaze upon, and discard the rest... Conan (o' Barbarian, not O'Brian) never had it so good. In hushed tones they spoke of the mystical property of "tightness". I noticed they never said anything about personality, sense of humor, whether the girl was happy, whether they were happy, whether they made each other happy. The only people I knew who were actually in relationships were adults who were always screaming at each other, so I knew exactly what I wanted: I wanted a best friend who was also a girl who would also do unspeakable things to and/or with me. The average guy sounded like he wanted a pet.
Okay, junior-high boys wouldn't know what to do with a real woman if they somehow magically got within ten feet of one. But - somehow, I literally do not understand this - some of them managed to hold onto this juvenile mindset into adulthood. It's like they really don't think of women as being complete people in and of themselves, with their own wishes and dreams and desires, they only see women in terms of their ability to serve men. And it's really easy for them to add other, specific, disrespectful actions or words to that larger, systemic disrespect.
Even if they can't see how disrespectful it is, or if they can see that but it doesn't bother them, I would think that they would eventually figure out how self-limiting that attitude is... at the very least their self-interest should have kicked in when they noticed that soon after they exposed this attitude most women ran for the door.
But some of them just can't (or won't) make the jump from junior-high boy to decent man without redefining what it means to be a man in a direction they aren't willing to go. I don't hate these guys, but I do consider them cowards who are too afraid to drop the false because (somewhere, deep down inside) they fear the truth is inadequate.