Why do men get mad when you question them about catching them looking at another woman's butt?

I have been dating this man for 4 yrs. I love him so much and I know he loves me. He says he is very confident in himself and does not have time for people that have no self respect or pride. There is this girl at work he checks out her butt all the time. But he says it means nothing. She has noticed him doing this and has recently been trying to put her arms around him and the other day she went up behind him and put her crotch and boobs up against hi s back side He told me about it and said that he moved away when she did this. We work at the same place, and I see some of the stuff she pulls. I asked him if he was going to tell her that he does and to like that and to please stop His response was "She does this with all the guys here... don't worry about it."I am not going to say anything to her I have to work with her" DO you know how much that hurt? Its like saying I have more respect for her feelings then yours! And when you say that you saw him starring at her they get all defensive and p*ssed off and tell you "no I don't" I can't believe you would accuse me of that" and then storm off and not talk to u. Why do they get so p*ssed off and deny it instead of saying ya I was ADMIT IT GUYS WHY LIE AND GET PISSED OFF.HE IS ALWAYS DOING THAT AND MAKING ME FEEL LIKE ITS MY FAULT AND EXPECTING ME TO COME RUNNING TO APOLOGIZE. I get so hurt and disappointed. So men answer this question PLEASE!