Proclamation of Teheran affirms the opposite, and most Western countries consider reproduction a right.
Go tell that to my friends who got married for visa reasons or to get a reduction in income tax. As for saying the benefits are a reflection of legal flaws, it’s like saying there is no such right to be safe in the streets since it does not involve any benefits, the few it does (right to protection from unjust harm) just being a reflection of the flaw that criminality is.
That we can agree on. But if you start giving that legal apparatus to heterosexual couples, as per people being equal in law, homosexual couples do have the right to marriage too.