I have wondered the same thing.....why marry if you are not going to be monogamous to each other.
After reading on my forums & egroups over the last several years it has come to my attention that a LOT of people don't realize they are poly until after the marriage. They marry, have kids together and then decide they wish to add some spice to life. They want a third (or more) adult to round out their family. They go unicorn hunting.
They discover that unicorns, while not totally mythical in the poly realm, are darned near close to extinction in the form they want it--to love them both equally (nearly impossible IMNSHO), the single HBB who is unattached AND has no kids of her own who will also be monogamous to them both (close the relationship), having no other relationships for herself.
Some have come to poly via the route of cheating. The one cheated on loves the cheater so very much that s/he is willing to open up the relationship for that ONE person so their spouse can be happy. Some even open it up to the extent that the one cheated on develops other relationships as well.
Still others have always had these feelings of poly-ness but bent to the will of society-at-large and decided to go the path of least resistance and have a monogamous marriage. They are miserable, not all but a good portion, and go one of three routes. They cheat, they leave their spouse so they can finally be who they feel they truly are or they talk to their spouse about opening up the marriage so they can stay together AND still have other loves and relationships.
Others have come to poly through self introspection. They have started dating a poly person, have done their research and come to the conclusion that it is for them. They decide that they would rather buck society and be happy with multiple relationships than go with society and maybe be happy sometimes and live a lie.
Others have always been polyamorous since their earliest years, never imagining anything other than having multiple relationships.
In short it doesn't always happen that a poly person gets married, knowing they're poly, and continuing to have relationships after the marriage.
Yes folks, this is MY opinion and MY opinion only. YMMV and probably will. I'm looking forward to seeing others thoughts on the topic.
There are as many ways to do polyamory as there are people practicing it!