Sexual revolution
Sexual revolution is the name for a number of behavioral changes that took place in the second half of the twentieth century, as a result of which the world became more liberal about sex. (Local differences existed in many countries.)
Birth-control pill
The United States, with a puritanical component in it from the beginning (the Puritans), tried to suppress birth control information harder than any other nation in the world as of that time. It became illegal in the latter nineteenth century to send birth control information.through the mail (or sex toys either, which in 2017 remain illegal in Alabama). The pessary or diaphragm, used casually in pre-World War II France by Anaïs Nin, was unknown. Condoms, if available at all, were "for the prevention of disease only"; they were never on open shelves as they are in the modern (2017) supermarket, but were behind the counter in phamacies and had to be asked for. The idea that sex might be fun, though often admitted in private, was all but anathema to the establishment of the time.
Into this unhappy country comes the birth control pill, in 1960. Now, as long as a pill was taken once a day, intercourse (penis-vagina sex) became carefree: no pregnancy! Contraceptive failure was far lower than with barrier devices. One need not interrupt or pause sex to put on a condom or insert contraceptive foam. One pill a day, by the woman, made sex pregnancy free, removing one of its biggest risks. Contraceptive pills did not provide STD protection, but this was unimportant because usually this is within a monogamous, or "monogamish" (term from podcaster Dan Savage: mostly monogamous but not totally) relationship.
Another reason the limitation on "bareback" (no condom) intercourse is less important than it seems at first glance is that it is well established that the sexually adventurous, such as swingers (wife-swappers), know the symptoms of sexually transmitted infections (STIs, more precise than STD, diseases) which display symptoms, and get tested for the ones which do not ( which? ). As herpes is only significant (serious risk) when giving birth to a child, it is very common among those that have promiscuous sex with strangers (porn actors/actresses, to start with).
STDs were eliminated
In the 1950s and 2960s, the only sexually transmitted or venereal diseases (as they called them at the time) were syphilis and gonorrhea. Sex ed. materials of the time often presented them as scary figures, out to get the unchaste. Then -- penicillin became generally available in 1945. Syphilis was soon conquered, gonorrhea was controled. No one but doctors had heard of herpes or sexually-transmitted hepatitus.
The reasons there were so few STDs is that there was so little Sex. A low rate of STDs means not much fucking was taking place. (Other types of sex existed, but they were a real fringe. The idea that cocksucking or pussy eating (daintily called "fellatio" and "cunnilingus") might merit serious attention from scholars, and be taught, was inconceivable at the time.)
Low STDs and low intercourse went together. If you don't have sex, you can't transmit a sexually transmitted disease. When STDs go up, that means there's been more fucking. (Fucking and intercourse (penis-vagina sex) were usually equated )
Pornography became legal
From the beginning, although the word never appeared in the document, it has been taken for granted that obscenity was not protected by the freedom of the press guaranteed by the First Amendment to the United States Constitution. The U.S. Supreme Court has clearly and repeatedly supported the position that obscenity has no First Amendment protection.
In the United States, where the judiciary makes policy to a degree unknown in other Western countries, the courts were used to change this situation. After a string of high-profile cases, some brought by the sleazy publisher of Screw, Al Goldstein, pornography turned out not to be obscene. Coinciding with dwindling support for censorship, because there was no definition of the obscene, literary classics (The Naked Lunch by William Burroughs, Tropic of Cancer by Henry Miller), then movies for which some pretense of "socially redeeming benefit" could be found. Works were to be evaluated "as a whole", not solely on isolated passages. It turned out to be impossible to objectively define obscenity, at least to the degree the U.S. court system would require.
Since pornography was not obscene, it was permitted. Porn was popular. "Bruce liked it." Books, in abundance in the 1950s-1960s (Greenleaf Readers), with competition only from relatively inconvenient and expensive 8mm movies, lost ground to hardcore 35mm porn videos shown in theaters in many U.S. cities. In a few theaters, such as the Park-Miller in New York, gay porn movies were shown.
What really made the porn industry take off was the video tape recorder, which became a consumer product about 1980. This meant that the photographic laboratory, the "developer" or "photo finisher," with its human operator, was no longer necessary. One could make one's own movies, and with two machines, make copies.
The 18 year age barrier
In the United States, one tool that the porn industry used to make acceptance
Condom fatigue
Phones became cameras
And system (Internet) capacity enabled massive transmission of videos.
Infection parties, and HIV+ men who give the "fuck of death" to unknowing, uninformed partners
Cuba - better life in sanatorium community.
Seaker of hepatitus C. A type of suicide.