I want to address the "Divorce Rate" argument since no one else has.
First of all, the divorce rate is on the decline. It spiked around the Women's Liberation Movement in the 60s, when many women felt empowered enough to finally leave their marriages behind. Since then, the divorce rate has stabilized and lowered; marriages now are lasting longer than marriages ten or twenty years ago. Secondly, the divorce rate for first marriages isn't actually all that high - the likelihood of divorce increases greatly with each new marriage, and these failed marriages get included in the statistics for all marriages. That skews the results a bit. Finally, the divorce rate still isn't that high. It's more frequent than most would like, but it isn't the epidemic that pundits in the media might want you to believe.
Also, divorce isn't a bad thing. Now, bad divorces can be heartbreaking affairs, but not all divorces have to be (or are) bad. 100 or so years ago, when divorce was still something of a taboo, if you were in an unhappy marriage, well, tough shit. You and your spouse had to suffer together for the rest of your lives, each resenting the other. Bitter fights would often be the norm, rather than the exception. That's not exactly a happy environment for children to grow up in.
Divorce is just a reflection of human nature - we all make mistakes. Finding someone to spend the rest of your life with is a tough choice - and hey, if you and your spouse realize that it isn't working out, things can still end amicably. People are no longer trapped or boxed in - and while growing up with two single parents may not be "traditional" for children, it sure beats the alternative.
So yeah. I know I've rambled a bit, but my points is that the whole divorce thing is a non-issue - or at the very least, not enough of one to justify condemning polyamorous relationships.