It seems to me like maybe you need to stop and evaluate your current situation.
How in love are you? I mean, to me it seems hard to believe that you could love this man so much and be ready to marry him, yet still wonder if there is someone else out there who could better for you. Is this cold feet or are you beginning to seriously question your relationship?
Cheating Is Not OK
I have so much I want to say to you. Cheating is never OK and never advisable. For one thing, you probably WILL get caught no matter how hard you try to keep to secret. This will happen because someone else finds out, because your behavior and attitude will change, or because if this man is really your soul mate, he will know that something is going on and he will question it.
You Are Young
You are young. There is nothing wrong with admitting that you are not ready to get married and that you may not be ready to commit to this one guy.
Maybe the answer is to sit down with him and discuss this. Maybe some time apart is necessary to hold things together in the long run. Maybe some time apart will show you both what you are missing out on either with each other or with others. Marriage is a huge step that should NOT be taken if there is a part of you that thinks that you are not ready for it.
You Are Not Communicating
If the sex is bad and you are feeling unfulfilled that means that you are also to blame. If you have not expressed to your fiance that you are unhappy or unfulfilled, how is he supposed to know? Remember, he is not psychic and he can't possibly guess whether or not he is turning you on and, if not, what he needs to do in order to be more fulfilling for you.
Overall, it sounds to me like the communication in your relationship may be lacking. I don't doubt that you care a lot about this guy but maybe he is not the one. The only way to figure that out is to be honest with yourself first and then with him. Maybe he is worrying about the same things- you never know.
It Is Human To Lust
Also, remember the difference between lust and love- without lust you are not human- everyone finds members of the opposite sex appealing and wonders what being with them could be like BUT that doesn't mean that you go after it necessarily.
Just keep reminding yourself that you are young and there is no reason to rush into anything. Now, before you get married, you still have time to figure out what you really want and where your fiance fits into all of it.
Maybe its just a matter of time for you to figure out what you want but you need to, above all, be HONEST and COMMUNICATE.