Context: I am a Bi Man with only one partner currently, and she has two other partners; a man and a woman (these would be called my Metamours).
How I feel about it is this: Relationships aren't made lesser or greater by the amount of them you have or lack there of, but by what they mean to you, what you expect out of them, and how well you all engage with those expectations. Monogamy is great for some, and Polyamory is great for others (personally speaking, a lot more people than are willing to admit it)
I don't want to exchange relationships for one another, just like I don't want to exchange friendships. For me, each person has different qualities and offers a unique experience or role that would be unfair to expect out of someone else. For example, my girlfriend has no intention of marrying me, or having my children, I know this and accept this in a Polyamorous relationship, and I get to appreciate the relationship I have with her in other forms. However, if I were in a Monogamous relationship, this would ultimately be a deal-breaker in the long run, and investing all that time and love seems like a big waste when you KNOW they aren't going to be your "One". This would be a terrible loss for me, because being with her has made me better and happier, and it wouldn't have been possible if I were Monogamous (since she had her other partners when we met)
Meaning no offence or disrespect to you or anyone, but in my opinion, Monogamy and its monolithic stance as the only way to engage with romantic, intimate, or sexual relationships, can be very harmful and toxic. Not all Monogamous relationships are toxic or harmful, they work for a lot of people; it is simply that in most situtations there is an expectation of universal adherence to the same set of relationship rules, that individuals may never talk about with their partner.
Having open and honest communication is key to any relationship, and polyamorous relationships are no different. In order for it to be truely open and honest, people have to feel comfortable approaching the topic of polyamory without the fear of reproach.
Thank you for reading, I hope I have been helpful, and that you have a great day.
P.S. The legality of something is not always the best point to draw the line at when it comes to inclusivity; gay marriage was illegal until 5 Years ago. I would apply that reasoning carefully.