the things men and women most want out of male-female relationships are very different. What women want is intimacy; what men want is good sex.
When both are willing to trade some for the other (that is, men are welling to allow a certain amount of vulnerability provided they get better sex for it, and women are willing to give sex in the hopes of intimacy), nether can find real satisfaction apart from the commitment of marriage.
Why hasn't the great experiment is casual sex worked? Marry Ann Mayo in her book A Christian Guide to Sexual Counseling contends that singles confuse sex with intimacy), "Scarcely does the person seeking intimacy through sex relate their [sic] emptiness, complication and health problems back to their source: sex with no commitment. Sexaual relations without commitment is like candy which tastes good but isn't good for you. You needs can't met.