First thing is you don’t let the left set the language of debate:
There is no such thing as “gay marriage” - just because they choose to call it “marriage” doesn’t mean it is.
Sexual relations between members of the same sex is wrong. Period. Giving them a “marriage” license doesn’t make it less wrong.
Also every man and woman has the right to be half of a marriage if they want. No rights have been denied. What they want is something that is not marriage to be called marriage. If I call the sun the sun and I decide to call the moon the sun are they now the same thing?