My point with birds is that in nature, pair bonding does not automatically equate to sexual monogamy - as we can see in the many, many human cultures around the world that have something that equates to marriage but that do not expect strict sexual fidelity. And as I said, even in cultures that do expect it, it doesn't truly exist - not even in cultures where the penatly is death. Because it's not natural - it's socially enforced in some cultures, but even then it doesn't really actually exist culture-wide.

For more on some of the cultures that do not practice strict monogamy, read this.

Marriage, Monogamy and the Nuclear Family Are Not Human Universals