It may sound like a good idea to you, but I know better. If it's going to be your first time, don't wait to be married. You're going to find out what you like, and likely it won't be with her.
I know high school sweethearts get married sometimes and stay that way for years. My first love went back to her first boyfriend in junior high, and they're still together today. They make the only couple I can use as such an example.
I had everything I ever wanted a while back. She was perfect in her attitude, beliefs, manner, and reasoning. We were going places right up to the point when she broke it off. The problem was all her, and it was in the back of my mind from the start. She was 21 and I was her second serious relationship. She came to the conclusion that I was the end of a part of life she wasn't done with, and ran like hell. So she's free.
Im sure had I been free a few weeks later, she would have come back, but only because I'd give her the truck back and with it some of the freedom she was looking for.
I loved her madly, knew what was going on, and immediately moved on. I didn't waste a day.
See, I'm 27. Had too many women come and go. Rule number one is no women under 25. I'm not about to invest my feelings in someone who still wants to run wild, who will simply toss me to the side when the time comes.