Lesbian fantasy?

I'm head over heels in love with my OH, we've been together a couple years, share a home and are TTC number one and have a wedding booked in 2016. Our sex life is great and I'm happy as I could be.  But lately I've been having wild thoughts about having lesbian sex, I find myself getting turned on and watching porn where a lesbian seduces a straight girl. Should I cheat on my OH or suggest a threesome with anther girl, I feel I need it out of my system before we marry!  Help please x