Religion didn't even become involved in marriage until the catholics came along and absorbed it. That is the first time a priest was required for a marriage.
Lastly, assuming you guys did invent it and you somehow hold a patent on it, why do you only want to deny gays the right to it? Shouldn't you believe atheists, muslims, buddhists and all other non-christians shouldn't be allowed to marry either? Why do they get to use your thing?