Bi curious and in a relationship

I'm currently dating the man of my dreams, but lately I have been fantasizing about having sex with a girl. We have talked about how I am bi curious and he says the only way that he would feel comfortable with me trying anything is if he is able to have a three some. Honestly I don't want to have a threesome. I just want an all out girl on girl experience. I can't tell him this though because I want to remain faithful to him no matter what. I just can't get the image out of my head, and it's making our own sex time not so much fun. I haven't cum in a week and trust me that's a lot, because we have sex two or three times a day. What do I do?