If you are against gay marriage, explain why without mentioning religion

[HoloDust]

Marriage is construct made mostly for purpose of procreation (as for other purposes...well, family comes from latin word for servant, slave) and giving most people opportunity to do it (why monogamous marriage is historically more accepted than polygamous). Marriage has not been invented for purpose of uniting man and woman in love, or with God, it is a social instrument that proved itself over the course of many thousands of years for goal of civilization/human species surviving and progressing. In it, offspring learns, among other things, their biological and social roles, so that they can continue cycle of civilization, and for that offspring to exist in first place you need male and female specimen (without recent involvement of modern science).

Changing definition of marriage defeats its main purpose - procreation.

That said, a lot of gay people contributed so much to our civilization, and thank universe for spawning them into this reality - but at the end, without male and female creating them, they would not exist in the first place, and marriage is the main vehicle of our society in which that happens.