What are your personal thoughts on polygyny or polyandry?

Humans don't have instinct.
Instintc= default conduct that all animals share.
Instinct of perpetuation of species? Not in humans! At least not in all, or else there would be no celibate people, like priests lol. Unless you think celibacy is a disease...
Also, people would never consider suicide if that was true, and people who suicide aren't sick either.

Instinct or innate behavior is the inherent inclination of a living organism toward a particular behavior.

It's our most basic responses to stimuli, that does not mean that through living in society we don't get to supress our instincts. It's what we are born with.
Take a human being and put it away from civilization and our rules. It will still seek to reproduce and survive, it's something that doesn't need to be taught,