The union of two persons of the same sex cannot be a marriage. The association of marriage with child bearing and rearing is not a triviality or artificial construct.
Not all unions of a man and woman will produce children, but no union of two persons of the same sex can do so. A lesbian couple on occasion has a child by artificial insemination, but the sperm donor is fathering a child while taking no responsibility for that child. Since he may be sharing sperm with others, this could lead to unintended acts of incest in the future.
Marriage has always meant a union of two persons of opposite sexes, and insisting on this doesn’t oppress or abuse anyone.