> Just for fun, it's worth pointing out that marriage itself is a human-created institution and not exactly "natural".
I would take it one step further and suggest that love itself is human-created.
I take "natural" to generally mean instinctual (raw emotional reaction), and as such any immediate (as opposed to a prolonged social feeling) reaction of sexual desire (lust) or pair-bonding instinct (nurture) can be categorized as "natural" - as opposed to socially sanctioned feeling-stories like love or institutions like marriage.