Most of the time, sex between opposite sex couples does not produce children.<quoted text>
I have no expectation what other self described gay men, or women do, or don't do.
And they can, plus ya don't need the state to do that! The state is not obligated to license every individual definition of marriage, if it was, polygamy would be legal, as would incest.
Why an individual couple chooses to marry is irrelevant as to why marriage is even recognized by the state to begin with. If not for the fact that sex between men and women makes babies, marriage wouldn't exist as we know it.
Many opposite sex couples can never have a child, no matter how much sex they have, while some can't even have sex.
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. Individuals who are 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.