Simple answer: YES! I went through this and there are times, 3 years after getting married, where I wonder. (Just this morning, I was looking through this group of nuns that I'm a fan of and was wondering if I was better meant to be a Sister.)
I have/had this really good guy friend, who a lot of people thought we were going to end up together. Even my Dad wanted us to be together (and he's never liked anybody else). But our paths didn't go that way and while I still wonder what it may have been like, I can clearly tell that being with him wouldn't be the right path God has set for me.
If this feels like a pressing matter and stays with you, then it might be something to seek out. However, there will be consequences and based on your paragraph below I wonder if this "other guy" is really worth it.
It might be just your type of personality where you are always wondering what if. I talked once to my husband about if he ever wondered about other women and he said no, it's never crossed his mind. I realize it is just that he and I have different personalities- I like thinking about each possible solution and what the outcome might be. I felt the same way about my husband prior to getting married- I knew he had faults and that I had other choices, but when it came down to it, I could not see myself honestly with anybody else. He and I fit so perfectly together that it felt wrong.
About a year before we were engaged, I did break up and started seeing someone else (a long time friend who I was interested in for a long time). For awhile I was really happy being with this person, but I noticed that we always hung out at his place and never mine. The first time he came over my place, I had a strong disliking towards the situation- it didn't feel right that he was in my space and I just decided I was uncomfortable about it since it was new. After some other stuff happened, I eventually came to realize how much I missed my ex and while he was really upset with me for breaking up and dating this other guy, he still loved me deeply and wanted me back. I didn't think he'd honestly take me back, but he only had open arms for me even before I apologized. I totally regret that relationship and us breaking up and wish it had never happened- but at the same time, it honestly made me ready to become engaged and get married. (Still I wouldn't recommend that line of action- my husband has forgiven me, we've moved on, but I can tell there's still pain there for him)
I think it's pretty normal- getting married is a huge change and is meant to be a permanent one. You can be the only one to decide if there are red flags in the relationship that would cause you to call it off or not. But I don't think you should consider other men when deciding this- it is not about other men, it is about you and your fiancee only.
The other thing too is that commitment is something we do each and every day. Sometimes we are really bad at it (maybe we got into a fight or was being selfish towards our spouse) and sometimes we are totally awesome at it, but I see commitment as more of a daily "baby steps" kind of action instead of this huge looming thing.
I know through various actions in our relationship that God was telling me to be with my husband. But each person's experience is different and I hope that you'll spend some time praying about it and discussing it with your fiancee.