Why is it illegal to have multiple spouses?

[Belacqua]

Why is it illegal to be a bigamist/polyandrist etc?

We're in the middle of a very long, slow move from values aimed at the social good to values aimed at individual good. 

This is what I was saying yesterday about how we are all soaked in liberal bourgeois values. Such values are in favor of individual liberty, with individual fulfillment as their prime good. This is in contrast to the values that Christianity tended to uphold, which were less focussed on the individual's wants and more on certain norms which were thought to be natural and unchangeable. These norms were given by God, but they were not arbitrary -- they were said to be for the good of all. 

Among these unchanging laws was that marriage was for the good of the kids (Thomas Aquinas says this clearly). It is not for the personal fulfillment of the individual. Of course he thought that it is the natural state for most people to be married (if they weren't celibate for religious reasons) so he thought that family life would also be fulfilling for people. But that isn't its main purpose. 

If we no longer accept that marriage is an institution for the sole good of raising children, and we adopt the liberal idea that each person should seek their own happiness in any way (so long as it doesn't harm another) then it's no longer reasonable to hold to traditional two-person marriage. If it makes 6 people happier to get married together, then our modern norms say we should go for it. 

Personally, I think that if your marriage is unrelated to having children, then there's no reason to stick to the old-fashioned ways. And it might be possible to have non-traditional marriages with kids too, if they are very carefully and unselfishly planned. A good pre-nup would probably be wise, though.