My question is if you want to have an open relationship, why even get married? Why not just seriously date each other, and fool around on the side? Seems like a better option. Personally, if you have children, I think it's incredibly unfair to them to be sleeping around with multiple people. Teaches them horrible values!
On top of that, what I find interesting about open relationships, is that at least from the open couples I know, they tend to be above average looking. I'm sure some of you will point out that you know open couples who aren't that attractive but my experience has been different. Which leads me to believe that better looking people know they can get lots of action anytime they want so they don't want to limit their options. But people who didn't get as lucky in the genes department, tend to value the stable relationships with one partner and expect each other to stay monogomous because they don't have as many options.
As to why they get married, I would guess it is primarily because there are a lot of financial and legal advantages to doing so (health care coverage for spouse, tax write offs, inheritance rules, etc). I suspect some still see it as vitally important that they make that commitment to each other that they are a UNIT for the long term. Also, not all of these relationships are two people together who "fool around on the side." The relationship in the OP is three people together as one stable grouping. For all we know those three would all be married as a group if there was some legal way to do so.
As to the teaching the kids horrible values--that is your belief on what is acceptable in a relationship and clearly not the belief of those IN polyamorous relationships.
To me, if the value kids are being taught is that you should take care of yourself and the ones you love and treat them with honesty and respect I do not care if the family that kid lives with has a mom and a dad, grandma and no other parent, two moms, two dads, Uncle and Aunt, jut mom, just dad, two dads, two moms, big sister or brother, grandma and grandpa, 2 moms and a dad, 2 moms and 2 dads, mom and stepdad or really any other combination of loving responsible adults heading it up.
Honestly, if it truly 100% something ALL the adults in a family want to be a part of and they all love each other and whatever kids come along--I can see that it might be really beneficial to children to have extra adults in their lives who love them, etc. Gosh--wouldn't it be nice to have 3 or 4 choices of who takes off of work on the day little Susie gets the stomache flu? Or 3 or 4 people to "divide and conquer" when Susie's baseball game and Johnny's belt test fall at the same time and that time just happens to be right about when Joey falls and cracks his chin open and needs stitches? Or even 3 or 4 people to take turns getting up at night with a colicky baby?