Mating Optimism:

"A period of mating optimism among young adult women may be a regular feature of female psychology. A woman’s reproductive value—hence her chances of marrying upward in the social scale—is at its height when she is young. These odds may favor the type of sexual restraint and concern with sexual reputation that would make finding such a mate more likely. As a woman ages, particularly if she experiences disappointments that suggest she is unlikely to get what she wants, a shift in mating tactics may be expected."

Elisabeth Cashdan