We've been dating for about 5 years now...probably proposing sometime this year. So I've got a little while til we do the kids thing.
If you feel this way after 5 years of dating, move on. It only gets worse. It's better to find a new one and spend less time making a decision to marry and more time getting to know the person. By the time you truly know the person you have married, you have reached an age of maturity that many of the things that bothered you as a young man, are not that big of a deal anymore. It sure makes a marriage less stressful.
If you really want to hit it rich, find a hot chick that has been treated poorly by an ex-husband and then just be decent. You will be so much better than the last guy that happiness will be spread around willfully.
But to marry someone after five years just because it is the "next step", is foolish. Been there, done that. Have the ripped/cut up t-shirts to prove it.