When did the Church first begin to unite same sex couples?

The first documented adelphopoiesis ceremony comes from 9th century. It's in Greek,
and it's published in John Boswell's book Same Sex Unions in Pre-Modern
Europe.

There are others of different periods, locations and languages. They
have minor variations but are the same rite. They date from 9th, 10th, 11th,
11th-12th, 13th and 14th centuries. Boswell quoted the original ceremonies
and translated them. None of his texts were unknown. But other scholars have
dug out more since.

So it seems that the ceremony is well documented from the 9th century. However it is referred to in various documents in previous centuries back to about the 5th century.

These ceremonies are from the Eastern church, however same sex couples were united in England, Ireland and most of Europe, wherever sworn kinship was practised.

However the customs differ in different areas and locations. Gerald of Wales wrote a parody of a similar church ceremony uniting two men, which he said was unexceptional in Ireland. However this does not mean that there is a lot of evidence and records of the ceremonies and the relationships they blessed have been actively suppressed until the second half of the twentieth century.

The adelphopoiesis rite was originally practised in a limited geographical area, but Boswell was not aware that it was practiced in his home country, the USA, at the time that he was writing. So the location may have a lot to do with where Boswell was looking, and less to do with where the rite was practised.