Some men are narcissists: they have no empathy for your feelings, they dont understand or care about the emotional turmoil they inflict on their partner. They will cheat repeatedly no matter how perfect you are. Narcissists have a very fragile ego and they see a new conquest as an ego boost. It also gives them a sense of power if the target is difficult to achieve (the trill of the chase) they may also use cheating as a way to punish their partner for percieved wrongs. Look up narcisicm and cheating
some men are insecure and they feel like their not good enough to get the type of woman they really want so they settle for second best and wait for something better to come along. They always think the grass is greener and are never happy with what they have. If they do manage to get their perfect woman-it just makes them even more insecure coz they are convinced they are not good enough for you, they think youll hurt them, cheat on them or leave them for someone else so they do it first to hurt you first, they may also do it to boost their ego, to feel more attractive, to look good to others etc
some men are emotionally immature. The use cheating as a way to escape from reality. Instead of trying to solve the problems in your relationship, they take the easy way out or escape for awhile. They cant handle reality, real life, real relationships, real responsibility. They could be the perfect partner when everything is good between u but then bail as soon as things get tough.
Some men lack the inner strenght necessary for a real relationship. They are weak. Again when everythings good they are great but they wilk hurt you during a rough patch coz they cant handle it.
Some men cheat as a way out of an unhappy relationship coz they dont have the balls to end it with dignity.
Some men are sex addicts.
If a man cheats on you-its not ur fault. He has issues-not you and you should just dump him and find yourself a better man. Someone who is confident, inner strenght, emotional maturity, integtity and emotuonal intelligence