Sex for pleasure as opposed to love or reproduction thought of as an ideal state of affairs. As Rousseau put it, 'the moral aspect of love is an artificial sentiment, born of social custom and celebrated by women with much care and cleverness to establish their ascendency'. Rousseau goes on to praise the Caribs for being much closer to nature. Unrestricted sex would create a better society he claimed, as people would be happier and less disposed to anti-social acts (which sounds reasonable).
Modern commentators might refer to the Bonobo Chimp society which is regulated by sexual exchange.
This theme was taken up by Diderot who attacked both celibacy and monogamy as evil. The Marquis de Sade took these views to their extreme, demonstrating that even 'immoral' acts could be sexual (though never suggested anyone should carry them out), thus giving a fully liberated account of sexuality, open as much to women as men.
Fourier was an enthusiastic disciple of free sex, sharing with Sade 'a sense of the power of the sexual drive and an appreciation of the diversity of individual sexual needs'. He did not take this as far as Sade, but he shared with Rousseau the idea that 'sexual liberation would not lead to the collapse of social bonds but to the reinforcement of social harmony'. In fact he was such a fan of free sex that he made it compulsory in his utopia!
Fourier's 19th century disciple John Humphrey Noyes, founder of the Oneida community was less authoritarian but called monogamous marriage 'a tyrannical institution that did not exist in heaven and eventually would be abolished on Earth'. He called instead for 'complex marriage' which today would be called Polyamory and insisted love should be a part of this, but equal love for everyone! Though he distinguished between 'ordinary love' and 'special love', coming down as hard on the latter as Fourier. This arrangement contained an element of sexism as it was considered against 'public spirit' for a woman to resist the attentions of any man. A crime that could lead to expulsion.
The turn of the century anarchist Emma Goldman had similar views, but addressed the subject from the perspective of women.
Alas even by the time technology, in the form of contraception, made this lifestyle choice a realistic one for the masses, sexism attitude had still not been overcome and attitudes amongst 'free love' male Hippies was in many was similar to Noyes. Though this gradually began to change in the 70's in some circles.
The 70's also saw a rise in 'free sex' that was more cynical than idealistic and promiscuity became the norm for a while. Since the 80's the combination of good common sense, dumb prudery and sexually transmitted disease has put a dampner on promiscuity. Today the nearest thing to 'free sex' commonly practised is probably serial monogamy (with the occassional affair), but Polyamory, of the voluntary kind promoted by Goldman, is on the rise in some quarters and in all probability will eventually become the next social norm.
Anykind of free sex is something that terrifies the repressed and prudish, something that will no doubt be proved by the down voting of this write up, unless I am pleasantly surprised. So read it while you can! | 
