It is definitely secular.

ir is neither secular nor religious. it didn't even start as an idea. some species are monogamous because that is what helps the species survive, and some species survive better polygamously. humans survive better monogamously, especially since our progeny remain helpless for much longer than the young of other species.

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Bible folk like kind David had has more then six wives and the traditional religion of Mormon promoted polygamy.

Secular? I don't know. This is an interesting read on the subject. [psychologytoday.com]

Marriage is really a relatively new concept for most people. Or should I say the whole hearts and flowers malarky. For most people, they just shacked up with whoever they liked. Marriage was more about property, wealth and the joining of families. It didn't really matter who the poor married. The OT has tons of variations on a theme and certainly right up until the 18th century most Brits had very simple ceremonies, if at all. I am not married for the first time since I was little more than a child and to be honest, I love it!

I don’t know. I’m sure some form of marriage has always existed. I think it will continue to exist where religion disappears. Sexuality exists on a spectrum so it makes sense that there will have always been both monogamous and non monogamous life partners.