Free sexuality definitely wasn't common in the 1920s! As I wrote in this previous answer, Did flappers use birth control?

Although there had been a dating culture involving trading sex for entertainment from the 1890s and continuing through this time among poor urban women, it's by no means certain that middle- and upper-class dates of this decade included a sexual component, since young women of affluent backgrounds did not require their male counterparts to pay for their entertainment to the same extent. ... The "amateur" - like the "charity girl" in the link above, someone who skirted the role of prostitute because she didn't take money - was a topic of concern, but she seems to have been of a particular class background.

It seems to largely fit with what I've also written about in this answer to teenage love in the Victorian era:

In breach of promise suits (usually women suing men for promising to marry them and then not doing so, but sometimes the other way around), it was typical for couples to only sleep together a couple of times in the latter half of a two-year courtship and engagement. They also had little opportunity for the amount of sex we now see as normal in romantic relationships: like elites, young people in this group were often with family or friends. ... In The Long Sexual Revolution (Oxford University Press, 2004) Hera Cook speculates that experiences of abandonment in the nineteenth century among the upper working class caused successive generations of women to be more and more suspicious of men who wanted sex outside of marriage, creating a culture of "respectability" by the end of the century that contrasts with the stereotype we have today of "prudishness" going along with the stuffy elite; she also notes that as urban communities settled in the later nineteenth century, the poor were better able to form strong social networks with more supervision and surveillance of the unmarried - but whatever the cause, the illegitimacy rate fell from 67% to 39% through the period, almost halving.

In context with another answer, I've read that it was incredibly normal for brides to be pregnant in the 18th century, but in Pride and Prejudice a couple's implied sexual contact prior to their marriage is a huge scandal. Is this indicative of a moral class divide, or were 18th century values just not representative of reality?

Intercourse while engaged was illicit, but much, much more licit than having an affair or a roll in the hay without any kind of precontract: marriage had previously been seen as based on the consent of the couple rather than the importance of the ceremony itself, as it would continue to be in Scotland (see: Gretna Green weddings), which may have been somewhat retained in conservative rural areas. In support of that, I'd note that said areas frequently also held onto marriage-mimicking courtship rituals that involved sharing a bed once in a while - fully clothed, encased in bags or body stockings, and/or with a board down the middle of the bed to prevent sex ... but these things could all be gotten around - or actual cohabitation!

Fiction, however, did not reflect this reality. Illegitimacy and pre-/extra-marital sex were certainly acknowledged in eighteenth century fiction (though they generally would not be in fiction of the nineteenth), but heroines tended to be strictly virginal. In part, this has to do with different standards for different social groups, as you suggested in your question text - the populations studied above are largely made up of the working classes, with individuals of the almost-upper-class social stratum of Jane Austen heroines (the landed gentry, the clergy, and the lowest nobility) representing a small portion of the whole that wouldn't greatly affect the percentage. Even the daughters of London tradesmen, solidly middle class, were said by social reformer Francis Place to have been less than perfectly chaste in the late eighteenth century without incurring social stigma. Where there was any pretension to gentility, though, virginity before marriage was the rule, which would include the middle classes by the early nineteenth century as possessing refinement became more and more possible.

That is, prior to the nineteenth century (in Britain and America, at any rate) having sex while being engaged/betrothed was fairly common among most people except the gentry and upper classes. Over the course of the nineteenth century, the elite standard of no intercourse until after the wedding spread downward, so that instead of this standard setting the wealthy off from everyone else, it set everyone else apart from the poor. By the 1920s, this was still true: most sex was likely happening in the context of serious relationships. The young women who were having sex outside of serious relationships were typically working-class, supporting themselves, and regarded as "delinquents" and/or borderline prostitutes (as they had been since this trend started in the 1880s). The 1950s saw soaring levels of illegitimate births to teenage mothers who typically would have been considered in that category as well. The world didn't become inherently conservative in the 1950s.

I am not an expert in historical drug use, but I would note that "lavish parties" have been a part of every historical period where there are rich people. There were lavish parties in the 1950s.