MY BOYFRIEND OF 4 YEARS CHEATED ON ME WITH A GUY...

So my boyfriend revealed he cheated on me on Friday. He said a week ago he met up with a guy he found off of Craig's List to have sex with. He drove a far ways away from where we live to meet this rando at his apartment. When he got there they got undressed themselves. He tried sucking my boyfriend off but he said he wasn't getting hard so he tried sucking off the other guy. He didn't like the guy's junk in his mouth so he stopped. Then the other guy put a condom on and tried to penetrate my boyfriend. He said he could only get the tip in before he stopped him got dressed and left... my boyfriend said it was the worst 5 mins of his life and now he's even more in love with me then he's ever been and he's so anything to take it back. He said something came over him and he felt like he needed to experiment. He wanted to know what it felt like. He wasn't aroused by the other guy and would never want to do it again... I don't know what to do... should I leave him? Should I believe him? In the very beginning of our relationship (4 1/2 years ago) that was my #1 rule. You cheat on me we are done because I've been cheated on before and it's broken me. I just don't know what to do... I still love him but now I can't trust him... HELP!!!!