I think there is a very simple non-religious reason against gay marriage;
Many forms of partnership exists. Think of room-mates, business partners, therapist-patient bonds and so on. They are valid and real. But they are not marriage.
Marriage is a particular bond between two people. It can incorporate producing children and physical intimacy, or it may not. There are marriages that no longer sustain these, yet are valid marriages.
Some marriages have no love. Some are marriages based on a particular need or convenience. But if they incorporate certain basic features, they are marriage.
I propose that marriage has been defined as: a relationship that provides exclusivity – at least on the woman’s part – and carries responsibility for the man, if for supporting the wife, or some other financial arrangement.
It refers back to the ideal of a woman and man living together, supporting each other and fulfilling each other. Even if the marriage is proposed for a certain time – say the pair intends to marry only for a year, – the frame of reference remains the institution just mentioned.
And it refers to a man and a woman. Nothing other than that is meant by ‘marriage’. Other relationships are called friendship, love or devotion.
So in short, the argument against gay marriage is that its not marriage. The word just doesn’t mean that, because it never did.