I think most people in there 30's can agree with this....when I was 18...I probably felt the same way about a girl....at some point everyone goes through that from teens on through mid 20's. I think it's a big mistake to make that big of a committment to someone at that unless you REALLY want to be married and want to start a family with that person. SO MANY THINGS WILL change in your life between 18 and 23,24,25. You will met so may people. You will change yourself. It would be a mistake to marry or seriously commit to someone who may change in a matter of a few years .....stay young and keep growing emotionally, spiritually and intellectually. I had know idea who I was and what I was about until my late 20's. It was then that I begin to meet people that are right for me and what I want/need in a relationship.
Good Luck!