Before we get much more into polyamory, let's question monogamy first. Monogamy is supposed to be one partner (sexual/emotional) for life. Society already dispels this. Most people date multiple people before choosing the 'one' anyway. And most people don't stick with one person for life, but rather one partner for a phase of their life. It's not usually intentional, but that's the way it often works out. People change, they grow, relationships no longer fit, they find new partners who do fit. Serial Monogamy is the norm in our society - or having multiple relationships over a lifetime, one at a time.  People who really do stay together for life are the exception, not the norm. 

So really, all polyamory is questioning about societal norms is : Why one at a time?

If you have feelings for someone new, why must you give up your existing, otherwise, happy and healthy relationship?  Or, as happens quite often, why is it more socially acceptable to cheat on your partner and have a discreet affair than to just be open and honest about it?

Polyamory is saying that just because you love someone new, that doesn't mean you quit loving the people you already do.... so why do we have to choose between relationships?  I mean, after all, a parent doesn't quit loving their first born when they have a second child, do they? And they certainly don't quit being a parent to one of the children, or hide the new child from the existing. 

I'm not at all saying that polyamory is the way it should be. Nor am I saying that monogamy is bad. I don't believe that polyamory is necessarily the best choice for everyone, cuz heck, relationships take time and energy and life if busy. Who has time for career, school, raising kids, taking care of aging parents, hobbies, volunteering and keeping up with multiple partners?  

But what I am asking is why is monogamy the only valid choice in our society? Why is monogamy the assumed default? 

If there were actually valid and accepted choices in our society for other relationship models, when someone actually *chooses* monogamy, that suddenly has so much more meaning over just taking a default.