Exactly! They have the same right to marry a person of the opposite sex just like everyone else does, so where's the 'discrimination'?
—so where’s the ‘discrimination’?—
We’re discriminating against what they “want” to do. Just like a brother and sister, or father and daughter, or a married person wanting to marry another married person.
All of the above, btw, are consenting adult situations. So, why not allow it?