yes it is normal and vary healthy to fantasize about other people if you do have sex with another person with out your fiancee's consent you are a scumbag though
There are several things to consider here. First of all, is this occasional fantasizing like once or twice a week or are you constantly thinking about other women? Second, you say you're just not as attracted to her sexually as you used to be. What has changed? Has she gained weight? Has she neglected her personal hygiene? What do you think is causing this? What has changed in your relationship lately? Have you had conflict? Are there people in your life that seem to be interfering with your relationship? Think of all the external factors that might be generating your lack of interest. Is the sex great? mediocre? bad? Have you tried to mix things up? Have you explored your sexuality? Toys? New methods? What have you done to make your sex life exciting? Every relationship requires work to keep it healthy. Yes it is normal to fantasize about other people besides your partner. I was checking out the appliance repair dude today and wondering what it would be like in the sack with him. I would never cheat on my husband. He is the light of my life but sometimes you get curious and explore mentally what sex would be like with someone else. You're a guy. That's normal for us. Once you figure what is causing the issue do what you can to remedy it. If need be contact a couples therapist. Like any issue in a marriage you can't let this stagnate. As time goes by it's going to fester and get worse. It will get to the point where you're going to do the unthinkable. You can't turn back after that. Once the trust is broken your relationship will never be the same. This is something you're going to have to work on together which means you will need to talk to her about it. But that's your decision. Relationships are never easy and they always need maintenance. If you really want to fix the issue then you're going to have to put in the time and effort. Make a list of the qualities that attracted you to her in the first place. Think back when you first started dating and what made you excited about her. Try to get that back. If you really think she is worth it then cowboy up and do your job.