I think there is still a lingering stigma in our society in relation to unmarried women. No matter how many times this debate comes up, and people make the point that women are equally capable of making marriage/relationship decisions as men are, we still don't get threads started by men complaining that their DP won't marry them and that they are worried that they don't love them enough as a result.
I do wonder how many women are unhappy about being married because they have this lingering feeling that it is what society expects of them. It has got to be easier to have an unfulfilled wish if you aren't being bombarded by advertising/TV/songs about how that particular wish is exactly what you should be wanting and expecting, or if you aren't constantly being quizzed by others about why you haven't got your wish yet.
I genuinely don't give a damn if anyone else thinks I should want to be married, or that I have somehow failed to "catch" DP properly, but then again I am not surrounded by people going on about it all the time. Our family is fine with our set-up and I work in a strong, all-female, multi-cultural environment - my colleagues all fly in the face of stereotypes in various different ways and I am probably the most conventional of the lot. So when, as happened the other day, a family member who really isn't in a position to judge on this issue randomly decides to quiz me about our "intentions" and question our motives, I can look at them like they are bonkers rather than feeling defensive about my situation.
If I had family, friends and colleagues constantly asking "ooh, when is he going to make an honest woman of you?" etc etc, I might feel a bit differently.
I am not in any way suggesting that this is the motivation of all women who are unhappily unmarried, but I think there must be some who would be less fussed about it if everyone around them was less fussed about it.