A text message to a former call?

My boyfriend responded to a call text from another girl by saying he shouldn't accept her invitation because A: he wouldn't want me doing it to him. B: He wanted to do right by me. And C: There was a possibility he WOULD fool around if he did go over. When I found out about this (on my own), he lied about it for hours, before he finally realized I knew everything, and THEN he confessed. Am I wrong to worry about him cheating in the future? Or should I trust him? His confession to another woman that he 'might' fool around if he were there-- is that a betrayal? And his denial of ALL of it as I continually tried to get HIM to tell ME-- can I trust someone like that? "?