It’s really very simple: marriage already has a definition and a function that has been observed, nurtured and promoted in every nation and/or culture on planet Earth throughout time and that definitely isn’t coincidence. Of what other custom or institution can anyone make that claim?
The definition is the key here because it provides the most unassailable base of logic, namely A is not transformed into B simply because someone says it is or wants it to be so.
The same goes for the rather bizarre argument that because problems such as divorce exist in marriage that married people are somehow ‘unworthy’ of its stewardship and that only gays can ‘rescue’ marriage. I hate cliches and motherly nagging but in this case two wrongs definitely don’t make a right.
Making a religious argument about marriage with an atheist is probably futile since they will be happy to attack religion all day in an attempt to redefine marriage. Ditto for religious objections to homosexuality, however justified.
For a nice head-spinner, ask him why he spends so much time attempting to disprove that which he is convinced is false already.
Marriage has definitely existed in (almost) every culture.
But the definition and form of marriage has varied quite dramatically, along with its social and legal implications and consequences.
What has never existed before, AFAIK, is anything resembling true “gay marriage,” but the variety of marriages between men and women is quite large.