Well, animals and children are not consenting adults capable of entering into a contract, so that’s a null argument.
Just redefine children to a lower age — the man love organization is attempting this very move — and give animals rights to engage in marriage too. Another tactic is to redefine marriage so that “consenting adults” is not included in the definition. This is not far fetched in that children were traditionally married off in different cultures without consent of any sort.