A Kiev art museum contains a curious icon from St. Catherine's Monastery on Mt. Sinai in Israel. It shows two robed Christian saints. Between them is a traditional Roman ‘pronubus’ (a best man), overseeing a wedding. The pronubus is Christ. The married couple are both men.
Prof. John Boswell, the late Chairman of Yale University’s history department, discovered that in addition to heterosexual marriage ceremonies in ancient Christian church liturgical documents, there were also ceremonies called the "Office of Same-Sex Union" (10th and 11th century), and the "Order for Uniting Two Men" (11th and 12th century).
These church rites had all the symbols of a heterosexual marriage: the whole community gathered in a church, a blessing of the couple before the altar was conducted with their right hands joined, holy vows were exchanged, a priest officiatied in the taking of the Eucharist and a wedding feast for the guests was celebrated afterwards. These elements all appear in contemporary illustrations of the holy union of the Byzantine Warrior-Emperor, Basil the First (867-886 CE) and his companion John.
Such same gender Christian sanctified unions also took place in Ireland in the late 12thand/ early 13th century, as the chronicler Gerald of Wales (‘Geraldus Cambrensis’) recorded.
Same-sex unions in pre-modern Europe list in great detail some same gender ceremonies found in ancient church liturgical documents. One Greek 13th century rite, "Order for Solemn Same-Sex Union", invoked St. Serge and St. Bacchus, and called on God to "vouchsafe unto these, Thy servants [N and N], the grace to love one another and to abide without hate and not be the cause of scandal all the days of their lives, with the help of the Holy Mother of God, and all Thy saints". The ceremony concludes: "And they shall kiss the Holy Gospel and each other, and it shall be concluded".
Another 14th century Serbian Slavonic "Office of the Same Sex Union", uniting two men or two women, had the couple lay their right hands on the Gospel while having a crucifix placed in their left hands. After kissing the Gospel, the couple were then required to kiss each other, after which the priest, having raised up the Eucharist, would give them both communion.
Records of Christian same sex unions have been discovered in such diverse archives as those in the Vatican, in St. Petersburg, in Paris, in Istanbul and in the Sinai, covering a thousand-years from the 8th to the 18th century.
The Dominican missionary and Prior, Jacques Goar (1601-1653), includes such ceremonies in a printed collection of Greek Orthodox prayer books, “Euchologion Sive Rituale Graecorum Complectens Ritus Et Ordines Divinae Liturgiae” (Paris, 1667).
While homosexuality was technically illegal from late Roman times, homophobic writings didn’t appear in Western Europe until the late 14th century. Even then, church-consecrated same sex unions continued to take place.
The evangelical Christian response will likely be to either question whether these were really like "marriage" or reject them as Satan-inspired evil that shows how far astray the Roman Catholic and Eastern Orthodox churches have gone.
Wikipedia has more on Sergius and Bacchus.