Many opposite sex couples can never have a child.<quoted text>
Still trying to equate a married couple who chooses not to have a child with a SS couple who can NEVER have a child?
Get real honey
They are allowed to get married despite that fact, as procreation has never been a requirement for marriage in any state or by the federal government. Even anti-gay Justice Scalia recognizes this fact of law. The elderly or otherwise sterile, paralyzed, amputated, or otherwise incapable of even having sex, are still recognized as retaining the fundamental right of marriage.
Your personal requirement of procreation ability in order to get married is your personal prejudice, not a requirement of law. Apply it to yourself if you choose, but the law does not impose it on anyone.