Why can't both exist

can be deeply intimate. I think there is something wonderful about having some aspects of relationship exclusive to indicate that a person is deeply special to me. I'd enjoy saying more, but am not really sure, more than trust.

Just my opinion.  Not an expert on this.

It seems to me that monogamy is talked about in terms of being a great spiritual commitment, and polygamy is not so much.  I don't think that both can't exist, but I think there's an important difference between the two.

The basis of my understanding of this difference is based on the idea of paternity certainty.  Let's not forget that sex is, at its most basic form, for reproduction.  Many species reproduce in different ways, but humans generally reproduce one at a time, which makes this thought easier to explain.

Every human being is mortal.  We all have a time limit before our deaths.  During this time, it is in our interests to maximize the reproduction of our DNA.  Living things don't survive by NOT reproducing.  We are designed to reproduce as much as possible and as well as possible... quality and quantity.  That means that each human being has a reproductive success rate which, for simplicity's sake, we can call Quality X Quantity, or Q for short.

Women never have to worry that the child coming out of her body may not be hers.  Men do.  Statistics show that 10-15% of children born to married couples in the USA and Canada do not belong to the father AND he is not aware of this.  What's bad about this is that the mother is not serving the reproductive interests of the father -- for every child she has that's not his, her Q value increases while his does not.  This invalidates a huge part of the value of the relationship for him.  Similarly, men who have other partners may impregnate them, causing their attention as a provider to be directed elsewhere.  This attention as a provider influences the Q value, as more attention for the offspring results in a higher quality rearing.

Enter conscious polygamy -- men can and do enter into agreements where they know that their partners may not bear their children.  Women can and do enter into relationships where they know that their partners may have children with other women.  There is nothing immoral about that.  There is, however, a lack of the possibility for each partner to be wholly serving the needs of the other.  If sexual activity is shared by one partner outside of a one-to-one relationship, the Q value for the other partner drops; for men, the likelihood that the child will be his decreases and for women the likelihood that her partner's attention will be taken away by a child other than her own increases.

It can be argued that if pregnancy is not the goal, then all of this is invalid.  I disagree.  I believe that we are hard-wired to be only as invested in a relationship as we need to be in order to maintain it.  This is called "social exchange theory".  If I value what you have to offer less than what it costs me, I will exit the relationship.  Additionally, we instinctively recognize what is and is not beneficial to our Q value.  Even toads and lizards defend their mates.  That means that if you make a choice which decreases my Q value, I will be forced to make a choice that decreases yours, and that means removing some or all of my commitment to you.

Only in fidelity and monogamy can this be avoided.  When both partners are fully committed to one another, the level of service to that other has a natural incentive to be absolute since increasing your Q value is literally the same as increasing my Q value.  Only in monogamy is it purely, physically, directly true through a process that sustains the very existence of your genetic code that the value of what you give is returned to you.  It's basically a karmic shortcut.  I view it as training wheels for spiritual realization; once you can see and feel how giving is getting, you can give in ways which bring you value less directly, knowing that the principle is the same.

I'm not saying that you cannot have spiritual connection through polygamy... I'm just saying that polygamy makes it near impossible for anyone involved to manage their own Q value along with those of others in a way that maximizes both quite like monogamy can.