So then, should heterosexual couples who are physically unable to produce children naturally not be allowed to marry?
The law would make perfect sense because no man would be allowed to marry any woman with whom he could not produce children with...applying to both heterosexuals and homosexuals.
However, your argument is not at all convincing that therefore, to be fair or some such thing, the state must rigorously step in and outlaw marriage between heterosexual couples who cannot or do not want to produce children.
These silly arguments have been answered again and again on these threads.