Perhaps to Kirk Cameron's surprise, marriage as we know it is not at all older than dirt. Marriage based on love is a very new concept. The Bible has nothing to do with the marriage we enjoy today.
The moment someone tells me "marriage has always been" something or another, I know they are ignorant of the actual history of marriage. It has never "always" been anything. It has taken different forms, with different social rules attached.
If neither Jesus nor Paul preferred marriage for their followers, why do some Christians maintain that the Bible enshrines 19th-century Victorian family values?
If marriage is good for society, and equality is the ethical basis for marriage, then gender difference is irrelevant. Marriage equality is good for everyone, including Bible believing Christians.