Why can't he can't stop fucking this girl?

I'm 17. My boyfriend and this bitch are both 18. Me and my boyfriend have been together for literally only like 3 weeks and been best friends since we were little kids. We grew up nextdoor neighbors. I've been in love with him for 2 years and I finally got the chance to ask him out since he became single again last month after his girlfriend broke up with him for cheating. With the same fucking girl. He's cheated on all his girlfriend's with her. I thought I was gonna be different. When I confessed my feelings to him he said he's always had the same feelings but he never told me because he was scared I wouldn't have the same problem or the want to be with him because of his cheating. But he promised me he would be faithful to me because of how he felt for me. Then last night I started walking over to his house since his parents were out of town when I saw him in his driveway with that same girl. I stayed low and watched what was happening. They were talking about something. She kissed him and they started making out but then he pushed her away and they started arguing. She went to kiss him again but he resisted and she put his hand on her butt and he hesitated and then started making out with her. And they went inside the house together. I went over to his living room window and looked inside and they were taking each other's clothes off. I didn't stay to watch the rest. Today he talked to me and acted like nothing happened and when I told him what I saw, he said he was sorry and that he told her that they had to stop and she didn't want to but she came into him and he could resist it. He "needed to do it" and lost control. Said every part of his body wanted him to do it and that he tried fighting it. What the fuck is so special about this bitch? I don't get it? I've been in love with him for so long and waited for him for so long and this is what I got? He said he felt the same way but he broke my heart already. Why can't he just stop with her? Why is she so special?
Why can't he can't stop fucking this girl?
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