should i stay with him?

So theres this guy, i always told him he liked my friend and he always told me no. So we started talking but it wasnt really that serious. as result he and my friend got drunk and had sex. I looked past it foolishly and kept talking to him. He told me time and time again theyre just friends. I took that as a he didnt like her and they ended sex. Now we are alotttt more serious and I find out hes still having sex with her. So i asked him and he said hes sorry and he knows its wrong. He told me they ended (like 2 weeks ago) and he still wants to be with me. I also found out she thought she was pregnant by him. Im hurt because i really really really dont wanna loose him, but idk if i would be a fool to take him back. PLEASE HELP ME