No, there isn't a good reason that same sex couples have not been accepted as valid.
A father or mother dying deprives a child of either father or a mother. Plenty of children are raised just fine without either of these in their lives. Some are raised better without either or both of their biological parents in their lives.
Just because a law exists, does not mean rights are not being denied. That is why we have a Constitution and a SC to enforce that Constitution.