Well you must admit your poll assumes that spiritual and scientific are some how mutually exclusive or dont overlap. Whether you're a scientist or a philosopher, you have to define your terms -what you mean by spiritual and scientific. The way your poll reads, you don't mean either. You seem to mean physical vs. emotional.
So what are you asking:
Are people poly because they believe it is more compatible with our biological instincts?
Are people poly because it gives them certain social or economic advantages, such as not trashing a marriage, disrupting a family unit, or selling a house?
Are people poly because they realize life is finite and don't want to miss out on certain life experiences?
Are people poly because having more than one person makes them feel more emotionally secure?
What is it you want to find out?