Conflicting opinions about same-sex marriage need not be contentious given the differences between civil and religious ceremonies. Civil unions take their authority from the State whereas religious ceremonies take their authority from God.
The religious form of marriage is also the original version, as outlined in Genesis, the first book of the Bible. Religiously married couples are graced as such under ordination from God. Civil marriages are something quite less; simply vesting rights from a secular state authority under a man-made law.
While true that certain religious sects are performing religious ceremonies for same-sex couples, this requires a tortured view of scripture, even a disregard for the word of God as he clearly instructs against homosexual behaviors. To conclude otherwise requires either ignorance, obstinance or outright rejection of widely known biblical teachings.
The simplistic beauty of this “argument” is that God himself settles the matter by disallowing procreation by same sex couples. Only a man and a woman can form a union that results in birth and therein lies the answer to the question of what constitutes a marriage in the eyes of the Lord.
As for non-believers, I guess for them anything goes and without rules, guideposts and firmly held beliefs, they’re free to make up their own definitions.