David Hume (18th century English philosopher) has this interesting little section on modesty/chastity in his Treatise of Human Nature. I won't quote the whole thing (for those interested, it's Book III, Part II, Section XII "Of Modesty and Chastity"), but here's a quick paraphrase:

A stable bond between man and woman is needed to raise a child. But before any of that, the parents must be sure the child is theirs. Of course, the woman can never doubt that it's hers; but the man is uncertain of this in proportion as his wife is unfaithful. So while there are other reasons that obligate both sexes to fidelity, this particular fact applies only to women and results in the greater emphasis placed on female fidelity than on male fidelity. In short, female infidelity, considered as a general phenomenon, has a greater potential to cause social disruption since it more directly undermines the basic assumptions that a family is based on.

Given the natural physical urge toward promiscuity, Hume sees in female infidelity a great tension between natural human passion, and the behavior required to maintain social order. He approves of legal repercussions for infidelity, but adds that this alone isn't enough. Social pressure, i.e. shame, is needed to further discourage infidelity. But still, both of these negative incentives are somewhat distant, coming only after the offense. The physical urge itself is much more immediate and therefore often wins out against the more abstract expectation of legal/social repercussions. And so Hume says that it is necessary to inculcate a certain emotional aversion to sex, to educate girls in such a way that when they become women, they have an immediate, innate apprehension regarding sex to counterbalance their physical urges.

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So there's Hume. Obviously all three of the barriers mentioned above are pretty weak today. As far as I'm aware, laws criminalizing adultery are either non-existent or unenforced. If your spouse cheats, you may get some advantage in divorce proceedings, but that's it (correct me if I'm off here). And frankly, being of a rather libertarian bent, I don't really know how much I want the government policing who fucks whom. Marriage is kinda a different case though, since it's a legal contract and should have repercussions for violating certain terms. As for the social pressures, these are really pretty weak now. "Slut-shaming" is usually more chastised than slutting itself, even though this very slut-shaming is, by Hume's logic, actually a beneficial force for social stability. And as for education, well, you see younger and younger people being fed tripe about how there's no shame in sleeping around, hell, it's even a positive assertion of your independence. Also, the notion that you should keep sex "hush-hush" around children is eroding, and this is exactly the sort of thing Hume was saying should be in place to provide an emotional counter to physical lust.

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Altogether it comes down to a shift in public morals. Now as far as I'm concerned, these morals are nothing but custom and should be evaluated on no other basis than how well they maintain a stable society. And in this light, modern sexual liberation type ideals just don't measure up to more traditional sexual morals. What do the first get you? I guess you get to feel more "free" for a while...? I'm really having a hard time seeing the benefit, beyond the hedonistic, immediate pleasure (which I hardly need to say is no stable basis for public morals). The traditional morals are just outright better socially speaking, primarily because they bring about more stable families. And then there's a whole argument to be made about how they are more beneficial/long-term emotionally satisfying from an individual perspective, more than even total sexual license. But I've gone on long enough.