Why did this make my boyfriend laugh so much?

We have been together for approximately 4 years and have lived together for 2. I have a strict rule for myself that I can't get too comfortable around him and always want to be "sexy" and "cute" for him. I'm basically a girly girl and love dressing up, wearing makeup, and doing anything romantic with him. That obviously includes no burping, farting, or using the bathroom around him because I think that takes away from my appeal. Well just the other day I was having a conversation with him on the couch while we were starting up a movie and I accidentally let out a little toot. It was not loud but it still was heard by him & there was nothing I could say or do to deny it lol. I was soooo pink and couldn't think of what to say but he just looked at me with a little smirk and died laughing. I was laughing with him because I was feeling really awkward. I seriously was so embarrassed and said "Stop laughing ahaha." He said "I can't help it! I've never heard you fart before this. You're not no princess anymore." He's more of a serious guy so the fact that he was laughing so much was embarrassing even more. Why did this make him laugh especially hard? More importantly, how do I stop being mortified by the whole incident. He's picking me up today from work to go out to dinner and I still feel residual embarrassment aha
Why did this make my boyfriend laugh so much?
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