Any sources for this being true in medieval Christian Europe?

If I can ask a follow-up, how is sexual activity in historical societies assessed? I assume relying on primary written sources such as diaries and histories provides poor data, but what else is there?

The tradition of Islamic law developed in the Middle Ages permits--in fact, encourages--sex in two cases: marriage between a man and a woman, and between a man and his female slave. Quite a few restrictions develop around the latter case, including protections that immediately accrue to the concubine (especially if she bears her owner's child) and the fact that the slave must be his; he cannot legally make a concubine out of his wife's slave.

So much for prescription. What about practice?

With respect to the initial Umayyad caliphate, as based out of Damascus in the 7th-8th centuries, there are two key things to keep in mind. First of all, at this stage, it's hard to talk about "Islamic law." Things are still very much in flux. Norman Roth, in fact, has argued that Islamic ideas of proper sexuality didn't really coalesce until the ninth century.

Second, while Arab/Muslim-led armies conquered a lot of territory fairly quickly, actual conversion to Islam was a much slower process. Especially in rural areas. For a variety of reasons, many of the medieval caliphates did not place a premium on proselytizing conquered peoples. (Isn't being able to levy extra taxes nice?)

So under the Damascus Umayyads very early on, you have nebulous ideas of restrictions on sexuality beginning to take shape among a small conquering middle/upper/military class, and a population maybe absorbing some ideas of Islam but probably still adhering to their prior practices. Whatever the existence of prostitution or affairs or sex with slaves or teenager rolling in cedar leaves, it's tough to say it would be regulated by Islamic law.

But despite the fall of the Umayyads in the east, the dynasty persisted in the west: the Caliphate of Cordoba, that is to say, medieval al-Andalus or modern-day Spain. And here things get interesting.

While there are beautiful romances that place their characters into adulterous love (a la the western Arthurian tradition) coming out of Persia, the vast majority of medieval Arabic sexual or erotic love poetry comes out of the courts of al-Andalus. Here I am talking about the Umayyads, and also the individual "taifa" kingdoms set up after the Umayyads' fall in the west as well. Sex was certainly "in the air" there, at least among poets. (Paralleling/intertwined with, to no surprise, the development of the romance tradition in Christian Europe).

But Layla and Majnun of the romance tradition die apart, their love unfulfilled; poetry is just words. Were the Muslims of al-Andalus having sex outside marriage and licit concubinage?

We can look at court cases, decrees and laws (with some skepticism--for the Middle Ages in particular, it's not always the case that "the existence of a law against it means someone did it"), and medieval Islamic legal scholarship to get some idea here. Thanks to the nature of the Arabic sources, most of our evidence concerns prostitution. The other important disclaimer is that the sources still pertain mainly to urban areas, and mostly to the upper classes.

At the level of general city life, we know there was the usual amount of street or brothel-level prostitution. Women, but also "boys." Weddings in particular were hot markets for prostitutes. There was a close connection between the singing and dancing female entertainers (so-called "singing girls") and prostitution, in rhetoric and apparently in practice as well.

As to concubinage. It is expressly forbidden in Islamic law for a man to pimp out his own female slaves as prostitutes. But that definitely happened, especially among the lower nobility or merchant class when they got a little low on cash.

Outside of prostitution, there are some surviving court records from inheritance lawsuits that provide concrete evidence of specifically premarital sex (as opposed to simply nonmarital). Children born prior to the marriage are illegitimate and do not inherit property; children born within the proper context can and do.

Additionally, authorities did take steps to set up segregation of sexes, with the need to keep women separate escalating as you moved up the social scale. (We can see lower class women appear in court cases more often than upper women, for instance. Under medieval Islamic law, unlike the Christian ideas that were developing in the later Middle Ages, women were full legal persons with rights such as self-presentation in court, not under male guardianship.) And everyday Muslims took their steps--sometimes literally--to get around them. Everyone today loves to talk about veils, but here's my favorite, from a 10th century hisba treatise (written as a Q&A with an Iberian/North African scholar):

Yahya was asked about a kind of sandal or flip-flop. Are shoemakers forbidden to make them? Because women look for these mules on purpose, with the intention of wearing them when going through the markets. And it so happens that, if a man is absent-minded, when he hears these chirping sandals he raises his head.

The point is not that sandals are sex, of course. Rather, people found a way to get around the religious laws and attempts to enforce them. Other hisba treatises number out the locations in cities where people were known to meet for sex: certain markets, river banks, orchards, cemeteries.

And, in another wonderful moment of history, the 10th century Cordoban jurist Muhammad bin Ishaq ibn al-Salim ruled only that the authorities would 'keep under supervision' a particular mosque known as a refuge for certain men and 'women of dubious morals'.

So overall, yes, people were having sex outside of marriage and legal concubinage in the medieval Islamic world.

Did major events such as war or crop failures increase or decrease this -- as in, if there's any way of knowing, was it used to deal with stress, celebrate, neither or both?

This is a very interesting question I'd like to see answered. However, could someone explain who the Umayyads are?

One factor to keep in mind here is that marriage was rather late (i.e., early-to-mid-20s on average, at least among commoners) in much of Europe. John Hajnal's original work concerned the divide, primarily, between eastern and western Europe, not the Arab world. That said, so far I haven't found anything about the Arab world in this period as a region of late nuptiality.

If the gap between puberty and marriage was smaller in the Umayyad Caliphate than in, say, the Plantagenet England, there might have simply been less time for premarital sex.

This doesn't really answer the question of how widespread premarital sexual activity was either in medieval Christian Europe or in the Umayyad Caliphate, but it is a factor that could be considered.

Muslim here, premartial sex for Muslims is STRICTLY prohibited, as unmarried partners can after sex maybe split apart, because it is not guaranteed that they will stay together. After marriage tho, you can assure that you'll stay together for at least two years (unless you're a celebrity). My point is that you could lose your virginity to someone who isn't your spouse. You can be used, so its just the extra step before sealing your fate with marriage.