The key to answering the claims of same-sex marriage advocates is understanding the basic rule of justice: Treat equals equally. If parties are not equal in a relevant sense, then there is no obligation of justice to treat them the same.
When someone demands the same recognition for homosexual unions as for heterosexual unions, remind him of the rule of justice. Then ask if homosexual relationships are really equal with heterosexual ones. Does he actually believe there is no essential difference between the two? If there are essential differences, then the two are not equal and there is no moral obligation to treat them as if they were.
Further, since legal marriage is really nothing more than officially granting public respect and approval to a union of two individuals, and since you as a member of the public do not in fact approve, then you have no obligation to support homosexual "marriage," and can even oppose it.