A Biblical marriage is one man joined in holy matrimony to one woman, and that the only sexual relationships that are acceptable to God are performed within the boundaries of a Biblical marriage. Any marriage, union, or sexual relationship that does not match this description is a sin and an abomination in the sight of God. These would include but are not limited to homosexuality, lesbianism, transgender, transsexual, bi-sexual, adultery, fornication, pedophilia, bi-gendered, cross-dressers, and bestiality.
Matthew 19:4-6, Hebrews 13:4, Leviticus 18:22, 20:13, Deuteronomy 22:5, Romans 1:24-28, Leviticus 18:23