No, marriage should be defined as between a man and woman

A 'should' statement has to be followed by an explanation as to what the goal is (e.g. "I should study if I want to be successful at school"). For what purpose 'should' we therefore keep marriage between a man and a woman? If the reason is "because it is a tradition", then it is important to note that tradition changes. Going back in time, marriage used to be only for the aristocrats, then for parents to arrange, for white people only, and so on and so on... Just decades ago divorce was completely unacceptable. Society changes and it changes it's traditions with it.
If the answer is that only man and woman can bear a child, why is it that couples that are infertile can marry? Or couples who never decide to bear children?