This is not something I address often, nor do I have any intention of fully addressing it, but it's a pet issue of many liberals.
First off, Thomas Sowell has a refreshingly analytical column on the misnomer of "gay marriage rights":
And secondly, this is much more whimsical, but for those still hung up on definition issues as relating to gay marriage, please examine the following algebraic analysis. Note that "!=" is "not equal to".
X = male
Y = female
//Pretty straighforward so far, eh?
X != Y
//*Most* of us can agree on that...male and female are different inherently.
X + Y = A
X + X = B
//Male homosexual marriage...
Y + Y = C
//Female homosexual marriage...
A != B
A != C
So there you have it, an enormously trite and imperfect analysis but I'm sure that would make it fit in quite nicely with the majority of rhetoric being thrown around on this issue!