- Abraham had a wife who brought him the maid to have children with when she couldn't conceive.
- Jacob married both a girl and her sister.
- Both King David and King Solomon had multiple wives and concubines.
- Jesus didn't marry and preached that whoever did the will of God was his mother, brother or sister.
- Paul never married and wished celibacy for all Christians, unless they burned with uncontrollable lust.
- Paul wrote to Timothy that bishops and deacons should be monogamous.
Several other passages in Paul's books and letters imply a marriage of one man and one woman, but it doesn't appear to be a mandatory structure. If anything, instruction about celibacy or monogamy are firstly about a person being free to pursue Christian service without being distracted by domestic life (a facet of celibate priesthood that seems to be forgotten in common understanding of the vocation). In fact, Paul's writing about family life demands that anyone who does marry should be submissive to the others in the family. No wonder it limits a person's ability to be an activist for the Christian cause.
But what's the consequence of this for gay marriage in civil society? Nothing. Even if we disregard the texts from the Hebrew Bible and concentrate on passages advocating monogamy, we learn that they are addressed to Christians who wish to be involved in a church office, i.e. bishops and deacons. There's nothing in there to prohibit gay marriage as a civil arrangement. If gay people wish to live together and have families, there appears to be no secular reason to stop them.
Is there a religious reason? Let's put it another way. Is there sufficient reason to warrant enforcing Christian ideas on the wider populace? If there is then banks should lend and expect nothing in return; weapons should be rusting from disuse; national borders should be erased; every person should receive according to need; every person should give according to ability; and so on. Christians are called to live that way, but aren't called to legislate any of it for non-Christians.
I'm not going to leave this as a hand-washing, though. There is more to this than simply denying responsibility. Gay couples may well have common financial concerns (investments, property). Should we deny them access to shared control of these assets? Gay couples may experience illness and hospital access may be limited to families only. Should we deny one access to the hospital to visit the other? There are a host of privileges that straight married people enjoy, perhaps without being aware of it. Gay couples are acutely aware of the disadvantage. If we are to love all people, regardless of particularities, and especially people who don't share our worldview, then we should support gay marriages because of the privileges that they will grant to those couples and because of the pain that can be avoided for them in various circumstances.
Gay marriage can't be argued down on the basis of a biblical model of marriage because there are too many biblical models. Christian marriages can't be legislated on non-Christians without legislating a host of other principles. And Christians should show compassion to all people, whether in agreement or not, and therefore should support granting civil rights for gay people to get married.