I don't agree at all but I respect your honesty in that statement.Honestly, I have no problems at all with polygamy. If 5 people want to enter into a joint marriage, that's fine by me. Some of the laws regarding marriage rights and benefits would probably need to be reworked to take into account multiple partners, but I don't think that it would cause any huge problems.
But again you miss the point. You are falling back on another law to justify another. Putting animals aside, Children can have jobs which some require them signing legal contracts so that argument in preventing them wont work. Children make personal choices all the time.This is true. However, you have to keep in mind that there are valid reasons for not allowing certain marriages that aren't moral judgments. In two of the most common cases mentioned (people marrying underage and people marrying animals) the reason for not allowing them is because animals and underage people are not allowed to enter into a legal contract.
It again comes back to a moral decision that society made.