Sorry to ruin the fantasies that many of you may be having about buxom tavern wenches, burly viking men, cloistered nuns, and randy princes, but while the Middle Ages were indeed a time that included a whole lot of sex... sex in bed, sex in churches, sex in the streets, between men, between women, between men and women and other identities, in many shapes, positions, and forms, and every other permutation you can think of..... that doesn't mean that this was a time of free love...

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Part 1, Late Antiquity

The Middle Ages bore the mark of Christian notions of sexuality heavily. The transition between late Roman notions of sexuality, heavily colored by the widespread availability of sexually exploited slaves, and Christian notions of proper sexual conduct saw casualties in the more free expression of sexual content, especially among expressions of same sex relationships.

The defining change of the Roman Empire from Classical to Late Antiquity was the conversion of the government, if not the people at large just yet, to Christianity. Along with this change Kyle Harper argues in From Shame to Sin that attitudes towards homosexual behavior between men were fundamentally altered. Sexuality became far more constrained in its acceptable outlets, namely between a man and his wife, or at the very least a man and an unmarried woman.

Firstly, focusing on homosexuality, Harper argues that we need to understand same sex relationships between men in the high Empire are not necessarily the same as a free acceptance of homosexuality. Instead the "acceptable" forms of same sex expression in the Late Roman world were invariably between masters and slaves, not free born men (and I do mean men, women feature between very rarely and not at all in ancient discourses on same sex relations). Indeed, seducing a free born man into sex with a man was a severe crime in Roman Law even before conversion to Christianity, this was not the Greek approach between men that was seen in Athens and the like in the 4th century BC. Secondly the survival of these master slave relations in the Roman Empire was also predicated on the widespread sexual availability of slaves through brothels. This point is worth underscoring. Without the widespread sexual availability of slaves to their masters, through both private ownership as well as through the sex trade, there was no Roman approach to homosexuality.

When Christianity became the prevailing ideology of the empire in the 4th century sexual slavery and public prostitution were heavily frowned upon, as in "could result in public execution" levels of frowned on. Over the course of the next several centuries various emperors would outlaw or suppress both sexual slavery and same sex relationships. The emperor Theodosius oversaw the public burning of male prostitutes in Rome and later Justinian would pass harsh laws condemning the sale of people into sexual slavery against their will. He condemned people participating in prostitution as well but the prevailing idea was that if you weren't being forced into the life you were legally culpable, so non-enslaved prostitutes were also targeted by Justinian's reforms.

Part 2, The Early Middle Ages

Stricter laws on adultery also came about during this time, and the sexual purity of Christian clergy, holy figures, and even devout lay people was a perennial concern among the late Antique and Early Medieval Church. While the sexuality of priests and bishops was not formally restrained by required celibacy until well into the Middle Ages, earlier attempts at limiting it were present. In the Anglo-Saxon period of England royal law codes forbade priests from taking wives, men from taking more than one wife (or relations with their slaves/concubines), not that this stopped people of course. The English king in 1066, Harold Godwinson had at least one woman commonly understood to be his wife, and not his nominal wife. Charlemagne before him likewise has a relatively non-pious married life, and there are replete examples of such misbehaving monarchs. Indeed sexual depredation on slaves was still a concern in 1020ish when the Winchester Code of Canute the Great was passed and decreed that women raped as slaves were to be freed.

More restrictive notions of sexuality are also seen in Medieval penitentials, these are handbooks for priests/monks essentially on how to ascribe penance to sinners (they're actually significantly more complicated than that, but for sake of argument let's keep it simple)

These penitentials circulated widely across Europe from their origin in Ireland, and among the many transgressions that they deal with, sexual immorality is of course a concern. Many sexual crimes were dealt with by non-clerical figures, adultery, rape, and so on were often included in nominally secular law codes of the Middle Ages. The penitentials though get really specific. Like REALLY specific. Some of them deal with the standard stuff, sodomy, masturbation, oral sex, but also intercural sex (intercourse through contact with the thighs not genitalia), incest, sex in churches, sex on certain days of the week...

In fact, please consult The Chart of when its ok to get it on!

[above credit to James Brundage]

Part 3, consider the pagans

But that's all Christians right? Surely the big burly Norsemen and women weren't as constrained by such puritanical notions regarding sexuality...right?

Well sadly no. Same-sex relations among the Norse were hardly open or tolerated, see this answer by /u/sunagainstgold

Nor did women enjoy much greater freedom in heterosexual marriage, or concubinage....

Iceland and Norway both seem to have been a hotspot for concubinage prior to Christianity and following its arrival this left many women in a precarious state, but even before that their options were not ideal. Men could refuse to acknowledge bastard children, women could be tortured if they did not reveal their lovers, and the sexual exploitation of slaves was rampant. Women enjoyed few legal recourses against those who raped them (all legal suits had to be brought about by male family members) and adultery was harshly condemned as well and could even result in execution.