When Jesus had finished saying these things, he left Galilee and went into the region of Judea to the other side of the Jordan. Large crowds followed him, and he healed them there. Some Pharisees came to him to test him. They asked, “Is it lawful for a man to divorce his wife for any and every reason?” “Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”
I quote this portion to point out Jesus clearly points back to in Genesis where the creator made man and female, and that the man will leave his father and mother and be united to his wife. It is not a man going from his mother and father to another man, or a woman going from her mother and father to her wife. A man (male) going to be united with his wife (female).
Another area I would say that makes a clear point is in Romans 1 when the wrath of God against sinful humans is discussed.
Romans 1 24 - 27
Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. They exchanged the truth about God for a lie, and worshiped and served created things rather than the Creator—who is forever praised. Amen.
Because of this, God gave them over to shameful lusts. Even their women exchanged natural sexual relations for unnatural ones. In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed shameful acts with other men, and received in themselves the due penalty for their error. Furthermore, just as they did not think it worthwhile to retain the knowledge of God, so God gave them over to a depraved mind, so that they do what ought not to be done. They have become filled with every kind of wickedness, evil, greed and depravity. They are full of envy, murder, strife, deceit and malice. They are gossips, slanderers, God-haters, insolent, arrogant and boastful; they invent ways of doing evil; they disobey their parents; 31 they have no understanding, no fidelity, no love, no mercy. Although they know God’s righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.
There are other passages but for the saving of time, I think you get the picture.
I would ask them if they had read this passages I just shared and a few others including Corinthians 6, 1 Timothy 1, and Genesis 1 and 2 . I would ask them to carefully consider what they were saying and ask how it was at all Biblical. I would pray for people with that perspective, as I feel people commonly attempt to distort scripture from what it really says. I would step away from a church that was performing gay marriage. I feel like other sins that it causes separation in the lives of those who partake in it. Not that a pastor of a church, elders, members, or others live without sin. My involvement with a church is based on the aim to follow the Bible as closely as possible. What I find particularly difficult about this sin isn't that I feel it is worse than others, but in choosing this sin I feel that it openly shows the choice of sin over the choice of Christ. For them to be in a leadership position and to openly choose to live in this life of sin creates a lot of confusion. Ultimately, the judgement is not for me to decide which is worse, I leave that to Christ. I don't see how that would be Biblical if that was their practice and I would leave.
Another big problem in my opinion is the idea of being able to tell a church or a pastor they must do this. The Idea of Separation of Church and State seems to be more that the government is beginning to dictate what churches can and cannot do. If it becomes mandatory for churches to perform these Ceremonies, it will be one of the saddest days for true religious freedom or freedom at all. The fact that we are having this conversation reminds me of Kings 1 and Kings 2 when sin ran rampant through the land. People were following their false Gods and it lead to their death. It scares me to see what the world is coming to.
Those are my prayerful thoughts.