[Do the math]

Marriage is one of the oldest legal contracts ever devised by man. The purpose of marriage is survival. Without children there is no future. Marriage is for:

producing of children;
providing for children;
and protection of children.

This is why the institution of marriage has been respected through out history by almost every form of government. Thus by its purpose marriage, in principle, must be between a man and a woman. Sense homosexual couples can not have offspring, there is no reason for them to be married. If gay couples want the benefits of marriage, they may form a legal partnership. There is no need for gay marriage.

[Regal]

Marriage is for:
producing of children;
providing for children;
and protection of children.

Counter argument: lots of married people don’t have, or want, children. I don’t think this will be a convincing argument.