A few details about the 1970s, from someone who actually lived through them.
1. Glam rock was popular in the 1970s, but among teenagers? Mostly only the really wild ones. It is highly unlikely that it was well-known in the wizarding world. In any case, in Muggle schools of the time, a boy who dressed androgynously and painted his face would have been beaten to within an inch of his life. Most boys who considered themselves the height of Muggle fashion in 1977 were wearing polyester knit printed shirts and leisure suits, not eyeliner, butt-length hair and skin-tight leather pants. This was the age of the mullet, for boys, and “wings” for girls (see Fawcett-Majors, Farrah). This means you, Sirius “I wear Muggle clothes to piss off my Toujours Pur family” Black.
2. “Safe sex” came into being in the mid-1980s, after AIDS had killed off the people who were being “sexually liberated” in the 1970s. Condoms were for boys who couldn’t get their girlfriends to go on the Pill and not for boys who did boys at all. If you tried to get boys to use condoms they would laugh at you. The big STD (we called it VD back then) threat was herpes, which is not curable, but is also more annoying than life-threatening, and many people didn’t take it very seriously. Lube existed, for anal sex of course, but we didn't call it that. We called it Vaseline, or Crisco, or salad oil...especially when we were teenagers and that was all we could get anyway.
3. The “Sexual Revolution” was in full swing and girls who didn’t put out were treated as prudes or lesbians (which was not accepted then as it is today, except as a titillation for men). Much, much more so than they are today. While many people in the 70s didn’t lose their virginity until they were 16-18, there were people much younger than that who wouldn’t admit they were virgins, and a fairly high percentage of 13-15 year olds were Doing It. In 9th grade, I was kind of embarrassed because people often came to me for sex advice, but I’d never actually Done It.
4. Date rape was very common; a lot of girls were ashamed and afraid to admit that they had said no because they were justly afraid of being disbelieved and/or laughed at, and far too many boys believed that sex was a right. Have fun, angstwhores!
5. While it was fashionable to be “bisexual” if you were involved with the art or music scene, it was not accepted in general society at all. It’s perfectly reasonable, of course, for pureblood societies not to have a problem with homosexuality, for a number of reasons (the wizarding world doesn’t seem to be terribly sexist, either, despite the fanon)-but anyone whose attitudes were strongly influenced by Muggle society is likely to look down on it. (Sorry, this is one situation where the purebloods are much more likely to be reasonable.) You can get away with being bi if you’re David Bowie, but not if you’re an ordinary person. Movies and books with “gay” storylines were considered very shocking (we used to sneak around with books that would now be considered utterly tame, like Patricia Nell Warren’s “The Front Runner” which I suspect was read by many more 14 year old girls than gay men and which was in fact my introduction to slash), and gay snogging was not carried out in front of other people.
6. The problem of bullying in schools was generally not taken seriously and dismissed as innocent fun; there was no such thing as ‘diversity training’ or ‘respect training’. Both the Marauders and Rosier et Wilkes could have got away with much more than they might today. (Remember that when asserting that Snape probably wasn’t hurt very much by the treatment he received.)
7. For an unmarried girl who fell pregnant to bear and keep her child was considered much, much more shocking than it is today and it is quite likely that she would be expelled from school as a bad example. The right to abortion was fiercely valued by those who had fought for it and frequently taken advantage of. As one of the things witches have been famous for throughout the centuries is procuring abortions, and the herbs which cause abortion are well known, please stop it already with the “I’m keeping my baby (because I'm NOBLE, and good people don’t get abortions)” fics.
8. People in the 1970s did not say ‘groovy’ unless they were being insulting.
The long and short of this is as follows: When I see sexual politics in MWPP fics that depend upon the wide acceptance of ideas that were new in the mid-1980s, it becomes very difficult to suspend my disbelief.