That's the wrong question, because they do have a right to marry.
What they don't have, is the legal ability to marry a person of the same sex.
If the question is framed as you framed it, the argument then shifts to the right to marry, and they argue that they are denied a right that everyone else possesses.
That's why they are framing the argument in that manner, because they already know the answer.
The real fight is to deny them the legal ability to enter into the institution known as marriage.