Perhaps we should encourage them to read the Bible.
Marriage was instituted by God as a relationship between a man and a woman. “Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh” (Gen. 2:24). The Church does not have the power or authority to change this—nor does secular society.
My mother yanked us out of the Episcopal church 35 years ago. Their backs have been turned on Christianity for a long long time.