In modern times in many places casual sex is normal. It isn't unusual for people to have 5-10 (or more) sexual partners before they get married. Is this a modern phenomenon due to effective contraception, or did people in the past also have multiple partners? I'm not talking about kings who kept harems and concubines, I'm asking about the regular citizens.
Yes, people tend to enjoy sex. My favorite medieval examples are the 12th century Paris statutes to build walls around graveyard and lock them at night to prevent people sneaking into them for sex, and the mosques in Islamic Iberia that developed shady reputations as places people went for covert sex. Here's a longer take on the question:
To answer this question you'd probably have to be specific. Ancient Mesopotamia is going to be different than 1840's Germany.
Oh yes! Most definitely. There's an amazing book on Venice in this regard. It's on amazon; "The boundaries of Eros: Sex crime and Sexuality in Renaissance Venice". Which is 13th to 17th century Venice.
It is a thrilling read and I can't recommend any book more. Some highlights include many cases of young noblemen climbing through windows and being forced to marry their "victims", some dreadful sexism where women are forced to go on a walk of shame Cersei Lannister style or are shown to the public and in some few cases are even executed and my personal favourite the nunnery that was closed by orders of the Pope because it was for all effects and purposes a brothel (the daughters for whom there was no dowry often ended up going into a cloister or nunnery).
There is also en entire chapter on gay relations, which at the time was found to be blasphemy, bestiality and more.
The entire book is based on a study of court documents of the time. Seriously amazing work. The sheer number of cases the author saw and visited indicates a much larger occurance. After all these were the people getting caught doing something "illegal".
Edit: title
(If this isn't an appropriate answer for this subreddit, I'm happy to make it comply or remove it.)
As an addendum to the other excellent answers, it seems important to add that there were certain cultural practices in place in the UK and in England from roughly the 1500s to the mid 19th century that may have mitigated social stigma of pregnancy (evidence of casual sex) outside of marriage. This shows that it did happen, though it was likely rarer and with reduced partners (edit: on behalf of the women, anyway) than today.
Marriages in the UK specifically were not always preceded by official church ceremony among the lower classes, especially before the Marraige Act of 1754; often the fathers of the betrothed were able to agree socially, and often a public declaration of the couple could suffice. If the female of the couple became pregnant, her honor could potentially be saved by a quick wedding (especially in the earlier, 17th century marriages; this source book on John Edwards and his war on such things includes that the number children conceived out of wedlock in the American colonies were as high as one in every ten), which in the UK were often called "fleet marriages" or "anvil weddings" (this stems from a practice of marrying at blacksmith anvils in secret, though they were not necessarily over the anvil nor by blacksmiths most of the time).
In a paper by Gil Newton (draft seen here, bib. of the full published piece seen here ) includes that the reasons for these marriages were complex and not reliant on surprise childbirth, and while a majority of brides were not pregnant in the time period, "Rates of prenuptial pregnancy in England as a whole were higher over the period 1650 to 1750, with a fifth to a quarter of women prenuptially pregnant, and nationally there was a rising trend then and thereafter, except in the final quarter of the eighteenth century."
Young couples also participated in an old practice called "bundling" in the UK and American colonies, in which they would lie fully clothed and tied up in the parent's bed (almost always the girl's parents). This was to "try out" the idea of marriage in a way that didn't compromise the couple's virginity, but many couples did have children out of wedlock. If the couple who bundled did become pregnant, the fact that they bundled identified the father, and the couple could marry before the baby was born.
I wrote this article about bundling and another here about anvil weddings, which might be of interest (all with in-text cited sources, some of which I replicated above).
These were some ways "casual sex" could quickly turn into marriage, if a baby resulted, at least in the UK and American colonies in the 17th and 18th centuries. Though I'm not positing that it was the main reason these marriages occurred, it does show that casual sex existed and to an extent was a little tolerated, particularly among lower class families and if the result was a marriage of some kind.