They want the right to do something the rest of us don't have.
If marriage can mean anything, then it means nothing. People will marry their dogs. People will marry their brothers.
What do they want that the rest of us don't have? The right to marry a consenting adult of their choice?
What does marriage mean in a country where 50% of marriages end in divorce?
We're not talking marrying people they are related to, we're not talking about something that could harm other people, incest.
But two gay people can't produce children, they're not able to produce babies that have serious problems, unlike brother and sister.