Marriage, in the legal sense, is a contract. You are presenting a slippery slope argument where there is none. Man and dog, woman and car, are not valid arguments because the dog cannot consent to a legally binding contract.
If two men and three woman want to enter into a polyamorous contract, that is the legal equivalency of marriage, why should they be prohibited from doing so? Realize that all five persons must sign the same contract accepting all of the other four, so a single holdout screws the entire thing up. Divorce would become interesting as well (you can still only sign a single contract at a time, just like today).