What to think of this>?!?! Help please?

my boyfriend and i hsve been together for a yr n 3mos. I love him very much, we haave had our bad times in our relationship, one happenin about about 5mos ago. He has never cheated on me, but I broke up with him @ one point and he slept with an ex(whom i cannot stand). He said that they were initially suppose to meet up at Dennys n just talk as friends do but she ended up just gettin a room(sounds crappy) It hurt so bad when he told me. He told me that he regretted it and he still in love with me and wants to marry me. I took him back because I love him, and I know he hasnt cheated. Should I be worried about this or jus let it go? Keep in mind he had never cheated on me while we were together. He told me that he was not talking to his ex while we been together, (his ex obviously still has liked him over this time).