Why is female sexuality more flexible than male sexuality?

A new evolutionary theory argues that women may have been evolutionarily designed to be sexually fluid—changing their sexual desires and identities from lesbian, to bisexual, to heterosexual and back again—in order to allow them to have sex with their co-wives in polygynous marriages, therefore reducing conflict and tension inherent in such marriages while at the same time successfully reproducing with their husbands in heterosexual unions.