I feel the same way sometimes, "what would it be like being with someone else?" but then when I actually see my fiance and hang out with her, I never feel that way, and only want to be with her. Shes a huge part of my life that I would really miss if something happened. Dont do it unless you are sure that you no longer love her. Maybe try going to new places with her, trying different things in bed, whatever you feel is lacking in your relationship, try to talk to her about it. If she doesnt want to listen, then maybe she isnt "the one". But seriously, think before you do anything because with many girls, once you leave, you cant come back.