we are biologically programmed to procreate---there are very strong social/financial reasons to do so within a couples framework--
i think many of the women who are >30 who claim they don't want to be married are doing so because they are establishing the answer to the inevitable questions they will receive---the biological clock is real, and it cannot be denied-----i don't believe in the fallacy of women who don't want kids--it's like a guy saying he doesn't want sex----it's simply not reality, although many fail to realize until its too late--i do feel badly for those who cannot have for whatever reason.
I think the key factors for women is the kids-----if they can handle it without a significant other, then good for them---but marriage is seen as the gateway to kids, and that's the driving factor--by the way, i also do not distinguish between married or committed cohabitants, the key issue is that couplehood allows the women to have a better chance of having/raising good offspring.