The problem for those who assert biblical authority in support of traditional definitions of marriage is that one could with equal validity assert that the lending of money or certain kinds of haircuts are forbidden by God or that slavery and the subjugation of women are authorized by the Lord.
May He who is the Brahman of the Hindus the Ahura-Mazda of the Zoroastrians the Buddha of the Buddhists the Jehovah of the Jews the Father in Heaven of the Christians give strength to you to carry out your noble idea.