I'm jealous but i shouldn't be

No -.- omg

A guy that used to be like in love with me got a girlfriend about 6 months ago. Me & him stopped talking for a while, because she had threatened me to stay away from him. We recently started texting again and we picked our friendship back up. He told me today that last night he got really drunk and had fucked her. Before they started dating, he was against drinking & sex before marriage. When he was like obsessed with me, I just thought of him as a friend & still do. But it made me really jealous. I don't know why.