His best mate

I had broken up with my ex a month ago. we still loved each othand kept seeing each other and fking. I knew he might be seeing another girl as well because people had told me. I asked him about it and he said he wasnt. about 3 weeks ago his best mate strated talking to me he said that he always thought I was sexy and that we should get together somtime.. we meet up and fked… I was still fking my ex as well. this week my ex found out that me and his best mate had got together he said that he was going to ask me out again if he hadnt of found out… and that he stoped seeing this other chick when he started seeing me again. he doesnt care what I did he just cares that it was hes best mate. he wants to know why I did it… but I really dont know I still love my ex very much… I told him that I dont know why I did it but he said that wasnt good enough I dont know what else to say I still ant to keep him as a friend or have I blown it… what can I say that wont hurt him ?? please help… many thanks