What was the law of god that prohibited same-sex marriage?

Answer 1

There is no specific, explicit prohibition of same-sex marriage in the scriptures, but many religions prohibit it based upon sacred tradition.

(Leviticus 20:13) 13 "'And when a man lies down with a male the same as one lies down with a woman, both of them have done a detestable thing. They should be put to death without fail. Their own blood is upon them.

(Romans 1:26-28)  That is why God gave them up to disgraceful sexual appetites, for both their females changed the natural use of themselves into one contrary to nature;  and likewise even the males left the natural use of the female and became violently inflamed in their lust toward one another, males with males, working what is obscene and receiving in themselves the full recompense, which was due for their error.  And just as they did not approve of holding God in accurate knowledge, God gave them up to a disapproved mental state, to do the things . . .

(1 Corinthians 6:9, 10) . . .What! Do you not know that unrighteous persons will not inherit God's kingdom? Do not be misled. Neither fornicators, nor idolaters, nor adulterers, nor men kept for unnatural purposes, nor men who lie with men,  nor thieves, nor greedy persons, nor drunkards, nor revilers, nor extortioners will inherit God's kingdom.

Answer 3 (Islamic perspective):

The Islamic Holy book refers to the biblical story of Lot and his people who entertained such actions. Lot , a prophet of Allah, was instructed to warn his people that what they were doing was wrong and that they would be punnished for it if they persist. Afterward Allah sent angels to lot to inform him that lot and his family will need to vacate the village, the Angels were sent to destroy it. Lot's wife was also a casualty. Muslims believe that the prophets was sent to us with guidance on how to earn Allah's favour. Taking note of the Prophet Lot's message is what makes us not support homosexual relationships.