“To be clear, the vast increase in the number of single women is to be celebrated not because singleness is in and of itself a better or more desirable state than coupledom. The revolution is in the expansion of options, the lifting of the imperative that for centuries hustled nearly all (non-enslaved) women, regardless of their individual desires, ambitions, circumstances, or the quality of available matches, down a single highway toward early heterosexual marriage and motherhood.”