*sighs* It's definitely a toughie man. Let me tell you something from my own experience regarding the "not wanting to be tied down." I was in a relationship with a girl for a long time (maybe year or so), and yes, as you, I was a young male. I began to feel the same way -- feeling I was too young to be tied down -- knowing there were many other fish in the sea to be 'tasted.' This began to bother me. I had lots of respect for the girl, and I didn't know why I started to think this way. I began to ask myself, "why was I now starting to feel this way?" The answer came to me a while afterwards:
I was not happy with the realationship; something was missing. There is a whole lot more to a relationship then the girl/guy being super nice. If they don't stimulate you (sexually and/or intellectually) then yes, your mind is going to 'wander' and you are going to start to wonder what it would be like with other women. I think you need to ask yourself what you feel you are missing from the relationship. Then see if it can change (this may involve talking to her about it). You never know, maybe she feels the same way in some aspects.
I truly believe that if you were 100% satisfied in a relationship, the aspect of 'of not wanting to be tied down' would never enter your mind.
"I still find each day too short for all the thoughts I want to think, all the walks I want to take, all the books I want to read, and all the friends I want to see."
- John Burroughs