We both have the right to be concert pianists, but only if we can sensibly fulfil the purpose and function of that status. In the same way, who can marry ought to depend on the purpose and function of the marriage state.
A cynic might argue that marriage no longer serves any useful purpose since there is an inexhaustible supply of young legal or illegal immigrants to maintain society and take care of the needs of those in their dotage, in which case you might as well recast marriage as a religious contract rather than a legal status. Which (apart from anything else) might end all the arguments.
;-)
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
