if homosexuals can marry, then brothers can marry because there is no reason to exclude them
and
if brothers can marry, why not brother and sister?
because that union can produce a child
So should women around the age of 40 be disallowed to marry because they are at a higher risk of producing a baby with Down's Syndrome?
How about people whose families come from Africa, South or Central America, etc.? They are at a higher risk for Sickle Cell Anemia.