Why did he dump me, then slept with someone else?

[hugeSpecialist780]

My friends just tell me to screw him but it's so hard for me to move on? He approached to me, I didn't even want to be with him at first but when we started dating I fell for him. I didn't know much about him but people around me said that he seemed like a nice guy which he really was. It didn't take very long before I found out about his ''internet life'' I mean he followed tons of porn sites and girls on instagram which he wrote to and asked for their snapchats, he friended random porn girls on facebook as well. This made me feel like he wasn't being serious with me so I asked him about it and he said that he only wanted me and a serious relationship so he deleted those girls. Over one night he turns distant and dumps me a week later with no real reason, he told me not to create any drama when I want to know more. We agreed to just be friends and he kept me on facebook, skype and continued to invite me to his groups on facebook. He created a page for a stripper and invited me to be a part of it, he then slept with her and posted half nude pictures of her on instagram?! He knew I still had strong feelings for him when he dumped me.. I don't understand what happened? Why would he dump me like that then sleep with someone else? I met his parents and friends so they knew we were together.