This is from an earlier answer of mine

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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.

Firstly, 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.

These legal developments went hand in hand with Christian developments of the will and the relationship between sexuality and the will. Harper concludes that over time the Christian ideas and messages seeped into the broader society through a top down implementation of strict anti-same sex relationships that ultimately derived from the letters of Paul that were used by rabble rousing bishops in Constantinople for example.

Over time however this focus lost much of its impact and focus and as we enter into the Medieval period Roman law collapses in much of their former territory and it is more difficult to understand the approaches that these successor kingdoms (with the exception of the Eastern Romans) took towards homosexual behavior.