That's a legal argument -- and not too many decades before that, anti-miscegenation laws were considered perfectly legal, even upstanding.
Now try to address the ethical argument. It is inherently fallacious to say that a minority is not being discriminated against by virtue of being forced to behave as the majority does.
"The law, in its majestic equality, forbids the rich and the poor alike to sleep under bridges, to beg in the streets, and to steal bread." -- Anatole France
To add to that: the title of this thread is "non religious reasons to oppose gay marriage".
Are you seriously going to try to argue that it is good for gays and lesbians to lie to themselves and to others to get into loveless marriages, and possibly have children as well? Do you really think we are better off with more dishonesty, misery, acrimony and broken homes? Don't heterosexuals do a good enough job on those counts already?