My boyfriend is mad at me but won't tell me why?

So my boyfriend asked me if guys have ever offered me sex.
Answering honestly I said, "It depends on what scene you're at. If you are at a bar or club then yes people are looking to score." I continued, "I would never take that offer."
The only person I want to sleep with is him and I do not cheat.

Towards the end of our call my boyfriend wasn't acting like himself. He said he was fine and that he was busy thinking about something.
The next day he seemed troubled. He admitted that it was about me but ended the conversation saying that we already discussed it and denied still being bothered.
I never want to make my boyfriend upset so this is saddening ><

My questions are
1) Why won't he tell me what I did to upset him?
2) What do you think he is mad about?

Thank you for taking your time to read this!
My boyfriend is mad at me but won't tell me why?
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