If she isn't married by the time she hits 30 years old? There is something very wrong with her that prevents marriage.
It might be debt. I've seen this. No sensible man wants to take on enough debt to pay a house mortgage off.
It might be health. I've seen this. No sensible man wants to try going out with someone who can't enjoy going out (gluten intolerance, picky eater, obsessing over her weight, etc).
Chances are vastly higher that it's something whacked in her head though.
So if she's had "a few" boyfriends, and none have wifed her up as yet, then it's a pretty good indication that she's unmarriageable. Shades of the old days, when an 18yo girl was considered a spinster who was unmarriageable.
On the whole it doesn't take much to turn any halfway-sensible guy off a girl. Some examples:
Always picking fights.
Using you (say, for expensive dinner dates - or just as a handy living dildo).
"Where are all the good men?" Nah, forget it. You're not marriage material, you spoiled and retarded and crazy cunt.