BILL O’REILLY made a very accurate critical point: "if you’re going to stand up for heterosexual marriage and exclude gay marriage, you’ve got to do it outside the Bible."
The 2012 book, "Saint Paul's Joke", discusses this very fact. The bible has a specific law against bi-sexuality in male (nothing for females); there is absolutely no mention of homosexuality, or homosexual behavior (with the exception of the no male bisexuality). St Paul does require that those who wish to engage in sex MUST be married. Again, no condemnation of homosexual relations, but the mandate that marriage must precede sex... or practice celibacy (regardless of gender or gender preferences).
From the standpoint of the Bible, those who wish to follow Christian doctrine (as set forth by St Paul), MUST support gay & straight marriage as a prelude to any sexual activity.
It must also be noted the most Christians follow St. Augustine – who specifically refuted all the teachings of Jesus, Peter, James and Paul. Thus, Augustine refuted, and denied, the basis of the Christianity espoused in 40% of the New Testament. The Church has also set aside the Old Testament (Torah), as a basic tenet of its doctrines and teachings – though its anti-Christ types do revert to citing, almost at random, passages.