I just don't know how to feel...

I'll try and keep this as short but fully informative as possible. I'm currently with my high school sweetheart, we have been together for 12+ years and while we have had our issues we love each other very much. I also made friends with a guy through a mutual friend and mutual friend, my boyfriend and I have become very close. About a month ago I confronted my BF after I started to notice him and our friend flirting, for lack of a better word. My BF had never mentioned to me any interest in men before this so naturally I was confused. When I finally confronted him about it he told me that he had been passingly curious for a while. He also mentioned that he wanted any experimenting to be with our friend, a lot of it due to comfort. My BF has never given me any leave to worry about him leaving me or falling out of love with me. He has told me on multiple occasions that he respects my opinion and that if I'm not comfortable with it he won't do it. Regardless, I'm still plagued by doubt of his word and guilt that I'm not letting him discover that potential part of himself. I can't help but think things like if he is going to leave me, if he would rather be with our friend, why I wasn't enough for him. Even though I have expressed these fears to him and he has vehemently denied any founding for these fears, I'm still terrified that it isn't the case. Am I being a silly, jealous girlfriend?