YTA

All my points have already been said so I'll just say that it's kind of telling that from what I can tell you haven't disclosed which culture you're previously from, assumedly because such a practice doesn't exist in your past culture.

This might just be excessive speculation on my part but are you maybe hiding a past relationship or sexual history with your 'platonic' friend?

Even if that's not the case the fact that you seem adamant that you deserve a free pass because 'muh culture' shows how little you actually care about your bf's feelings. You royally screwed up and instead of trying to fix things or reconcile you've instead come here to get validation and to double down on your actions. You completely embarrassed him and then failed to respect his feelings due to said embarrassment, which is probably why he put down the ultimatum. It sounds like you care more about absolving yourself of sin then actually caring about your BF at all. I probably would have acted exactly as your BF did, and I think most people would have.

You are going to have to accept that you were in the wrong if you want any chance of reconciling.