When speaking of marriage as it exists today, it is a state run, or least a state mediated, institution. The state operates under a principle of coercion, violence, and kidnapping. So any relationship which is a marriage has an added element to it. That element is the violence which one or both parties may wield by proxy. This changes the entire dynamic of the relationship. It is now an enforceable contract. Enforced by violence. Denying this fundamental element does not change its nature.
Instead of asking if gay marriage should be allowed or not, the question that should be asked is how much violence is acceptable in a marriage? To me the answer is none. And that extends to either party involved in the marriage contract or to any other parties enforcing that contract. Removal of state mediation within marriage ipso facto legalizes gay marriage.