Sex Confession

I have been with my husband now for 9 years. We are both 26. I've always wanted a threesome with another girl and he knows it. As I've gotten older, the thought of him being with another girl makes me so uncomfortable but I still want to be. I'm straight but I've always got turned on by the idea of girl on girl. I still want to have sex with a girl but feel it's inappropriate since we're married. I just can't help but feel this way and I don't know if I should tell him or just keep it to myself.