The argument : Society needs traditional marriage to promote new families and children who will become the leaders of the next generation. Same-sex marriage does not promote this. Rather, it promotes the wants of individuals
The response : Ask if this person is really saying that marriage should exist only for the purpose of producing children? Many marriages produce no children at all, but are either childless - whether by choice, or due to infertility or age - or are home to adopted children. These marriages are no less marriages in the eyes of the federal government, or of God (ask, "Don't you agree?"). Those people are accorded no fewer rights because the two married individuals do not procreate - nor should they be. Many same sex couples either produce natural children or adopt, create new families, and are no more self-serving to the individual than any opposite sex marriage in which children are dearly wanted. The desire or ability to procreate should not determine which rights a married couple should have.
The argument : It's not about Civil Rights. Gay rights should not be equated with the struggle of African Americans for their Civil Rights, they were never slaves
The response : Nonsense. It is about Civil Rights. Civil Rights means the rights accorded to each and every citizen of the United States being equal and across the board to all citizens. It isn't just about slavery. It's about equality under the law of our land. That means African Americans and gays and women and anyone else who's being denied equality under our Constitution. Thankfully, the black Civil Rights movement has greased these wheels considerably, and provided a template so that other minority groups experiencing discrimination can achieve more equal treatment in decades rather than the centuries it took for African Americans to get as close as they have (though there's still a ways to go, there, too).
The argument : It's a slippery slope that will lead to incestuous marriage, or even bestiality in marriage. Just because a brother and sister are in love and want to marry, the law forbids it for good reasons. If Gay Marriage is allowed, then where will it end? Why not allow polygamy, too, for that matter?
The response : This is the most spurious argument of all, and it's just a ridiculous one. Gays want the same rights as straights. Incest will still be disallowed. Bestiality will still be disallowed. Marriage should be the committed relationship between two persons of consenting age which is lawful in all other ways. The "slippery slope" is all in the minds of these poor people - they are so afraid their way of life is being threatened that they are grasping at whatever straw seems to be waving their way.