Personal view here:
You're saying this to a guy that went through a bad break with his ex.
Ever considered this alleged new squeeze has seen the same thing? Marriage isn't as attractive as those who are entrapped in it tend to believe.
Entrapped? Marriage saved my ass, got me on the straight and narrow and I've gone further with my life and career than I ever expected. That of course comes with picking the right spouse, not some chick you knock up on Spring Break that looks good and is "perfect" when you're buzzed or a chick that has no morals, future plans, or integrity. Marriage ain't all rainbows and sunshine, but it's clear that it hasn't always been 100% roses with the women you mingle with either, so it's similar. You can walk away, sure, but I don't want to walk away is our difference.
Some of us would say you just haven't found a solid woman or one worth devoting your life to. Selfishness, maybe? No one should just settle though, so maybe you just need to find the right one. Also, marriage is a Christian act, so if you're not a Christian (not saying you aren't, just stating in general) I still have no understanding of why marriage even matters to those people.