Is it selfish to want to have sex with a woman when I am currently in a relationship with my bf?

Ok, so I have been with my boyfriend for about 10 years now. I started dating him when I was young. Everything is ok between us I love him and he loves me. But it has been a known fact between the two of us that I am bisexual. I didn’t start embracing or even telling anyone about my bisexuality until around the time I began dating my current b/f. Now its 10 years down the line and I cant get the thought of wanting a woman off my mind. Its like I am a virgin again, and it has been stressing me out. I have no desire to leave my bf, but I am also severely sexually frustrated. We have talked about having a threesome which I am fine with, but I can’t help the feeling that I will never be with a woman sexually one on one. I do not know how to talk to him about this. Like I love him to death and want to be with him, but I feel like this whole other side of me will never be full filled. Am I being selfish to want to have sex with another woman one on one? I just feel like I need to do this at least once to get it out of my system. I dont think my bf will understand this since he has no idea what this feeling is like. We have talked before and it has been said quite clearly that I am not to have sex with another woman on my own because it is cheating. I just dont know what to do anymore, and I cannot no matter what get it off my mind. I feel like it is just becoming so over whelming to where I am suffering mentally. That and I am so scared that I will screw up the relationship that I have.