Boyfriend slept with his female "best friend"

My boyfriend has a female best friend (she lives in another state). I just found out (after two yrs of dating) that they had sex in the past, before he and I were together. He lied to me about it and only admitted to it after I saw a message of her saying " remember yrs ago when we did such and such" (he never responded to this message from over a week old.) When confronted he did not apologize for lying to me because he said he did nothing wrong and only lied because it was from the past and none of my business and that they had slept together way before he ever met me. He said he lied because he valued their friendship but didnt want that to prevent me from dating him. I feel he owes me an apology. He claims he no longer talks to her because she disrespected our relationship by bringing up the past and that they never talked about their sexual past once it was over . I feel stupid for befriending her and not knowing the truth about their past. I am having a hard time trusting him now. I really just want an apology at this point and ownership of his lie.

Feedback welcomed