Biblical marriages not only were endogamous arrangements within the tribe that often included a bride price, but also were polygamous.
Readers in Genesis learn that several of the "patriarchs" had more than one wife. Abraham married Sarah and Hagar. Jacob married Rachel and Leah. Esau married Judith and Bashemath.
This cultural convention of the ancient Hebrews received recognition in the Mosaic Law. In Deuteronomy Moses provides the following directions for inheritance problems caused by polygamous marriages:
"If a man have two wives, one beloved, and another hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated: Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his." Deut 21:15-17 (KJV)
And, of course, later examples of polygamous marriages in Hebrew history include, Gideon, David, Solomon, and virtually all the Hebrew monarchs.
Polygamy is something we do not hear about when Christians talk about "biblical marriages."
We do hear about polygamy from Christians when they argue that state recognition of same sex marriage opens the door to polygamy. Once the laws rip marriage from its heterosexual foundation and childbearing purposes, they argue, then anything goes. No reasonable grounds remain for any definition of marriage.
This is sound as far as it goes.
Because polygamous marriages were a cultural tradition among the ancient Hebrews and recognized by the Mosaic Law in the bible, Christians cannot make this argument and remain consistent with the propositional truths contained in their collection of sacred writings.
Not that it is a bad argument. But unfortunately for the Christian, only we non-Christian supporters of traditional Roman marriages of one man and one woman can make it.