But they all are descended from exclusively heterosexual couplings, and never from homosexual couplings. Religion, law, culture and tradition have always encouraged heterosexual couples to marry because heterosexual couplings frequently lead to procreation. I suspect, if homosexual sex also lead to procreation, a similiar or the same tradition would have evolved for homosexuals.
And even though there is no longer the need to multiply to the extent there was when the bible was written, the benefit of children being born into homes with both their mother and father present to provide and care for them, and fewer born into the alternative of homes with one or none of their parents present, continues. Children tend to do better when they have the benefit of both their mother and father present to provide and care for them. Thus the encouragement of heterosexual couples to marry. Simply having homosexual parents doesnt present the same advantage of biological parents.