Because God's word is unchanging...

Wow! Did you know that the Christian church used to officiate at same-sex unions?

The most famous were St Sergius & St Bacchus

In the definitive 10th century account of their lives, St. Sergius is openly celebrated as the "sweet companion and lover" of St. Bacchus. Sergius and Bacchus's close relationship has led many modern scholars to believe they were lovers. But the most compelling evidence for this view is that the oldest text of their martyrology, written in New Testament Greek describes them as "erastai,” or "lovers".

[...]

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.

There's some controversy, of course, with more conservative commentators arguing this was simply a sort of rite for men (and one recorded case of two women, in Dalmatia) to formally adopt each other as 'brothers'. Ha!