We need to agree on facts about marriage.
It is often called, “The Institution of Marriage”. A more accurate term is the “Practice of Marriage”, or, just “marriage”.
Marriage did not originate as a gift from God. In tribal societies today, and presumably from the beginning of humans, it was a business or survival deal. Marriages were arranged by others for the benefit of the family or tribe.
Marriage is not for the purpose of having and raising children. This has to be the case because some marriages do not result in children. Some children are born to parents that are not married.
Marriage in the Bible was not universally between one man and one woman. It was often between one man and several women.
Marriage did not originate from religion. The early, and current, arranged marriages were negotiated by tribal elders or family. When churches came along they inserted themselves into something that had been established long before they arrived.
We must remember humans did not link sex to pregnancy until a few thousand years ago. For most of human history, it was believed that women created babies themselves through mysterious powers only they possessed.
Gay people are successful in raising children. There are conflicting studies about children of single parents and gays and others. None has been found to be unsuccessful in raising children.
Opposing gay marriage for reasons not grounded in fact is unethical. One thing we know is stable households, gay and straight, are good for all of us.