In most jurisdictions, marriage is pretty easy to get into. [Just a side note: in Texas, in fact, the state recognizes common law or "informal" marriages(along with 14 or 15 other states). You don't need a license or a ceremony... it won't be registered in the state's vital statistics registry or anything, but you are still very much married if you satisfy the three simple elements in the law.] The difficult thing is getting OUT of that marriage. Once you file for divorce, the state (via the family court system) starts to micro-manage everything in that failed partnership: property (real and personal), child custody, child visitation, child support.

How do you dissolve a marriage or civil union with three people involved? What about six? What if only one person wants out and the others want to carry on? Do non-biological parents have visitation rights to children who were born during the union but are not actually their children? FWIW, I do see some wisdom in limiting civil unions to two adults.