Why are guys abusive?

iv been on a couple dates with this guy and he was really nice and cool and so i went over to his house and he was drinking and walking around with his thing hanging out and trying to grind up against me and touching me and i told him i was leaving and he shut the door, grabbed me, put me on the bed and started grinding against me , he would let me up and i wasnt strong enough to force him off me and after a while he let me up and i left really shaken up. he told all his friends what happened and now every time they see me they grab my *** or brush up to me inappropriately at school and pubilc places. what should i do about this. and why are guys so abusive and contolling of girls :(