Unsure on whether or not to come out

So okay, I’ve been with my boyfriend for 6 years and I plan on being with him for the rest of my life. I’ve never dated a woman, I’ve kissed a girl back in high school, though I’m not sure that’s relevant. Anyway, I’m bisexual and I’ve never told my boyfriend. I don’t think I realized I was actually attracted to girls until after we were together. My question is, I don’t know how or if to tell him? I mean, I don’t see myself ever dating a woman considering I’m with him. I just feel like I’m hiding a part of myself to him and it feels weird. No one else in my life knows either. Would you tell them? If so, what would you say?