Why didn't he tell me?

Kaylee

So last night my boyfriend and I were having a serious conversation and he mentioned how he'd never cheat on me, he's had the opportunity and he didn't. And I was like what do you mean? Why haven't you told me. So he proceeded to tell me the story.... then I realized that he told me the same story once before but he neglected to mention the part about the girl who was laying naked in his friends bed waiting for him. I know he didn't but why didn't he tell me in the first place??? It just upsets me because although I know he won't cheat on me I don't trust him (I trust him but I don't because he didn't tell me!) Does that make sense? What should I do?