People who are unhappy in their marriages are people who get married just to be married. Just so they can tell all their friends and family that they're married and are "part of the club" now.
Marriage is something you do when you're ABSOLUTELY sure, not before.
Our society tries to force unmarried couples into marriage because there are so many people who got married when they weren't absolutely sure they wanted to be, and they try to justify it to themselves by saying, "Hey, everyone around me is married!" Thus, they try to convince all their friends that married is the way to be, to feel better about themselves.
Anything that you do because you feel that you HAVE to do it is less likely to last than something you do because you CHOOSE to do it. DOn't let anyone...your girl, your family, your friends, ANYONE, pressure you into such a life-changing decision. That decision should be YOURS alone and you should do it not because you want to get married, but because this girl IS the only one you'll ever want to be with again and you can honestly look yourself in the eye and say, "if I coudl have this girl every night for the rest of my life, I would be perfectly content giving up all others."
If you can't do that, then don't get married. No matter what ANYONE says. It'll just make you both worse off.