What part of this is not perfectly clear:
“they knew their grandchildren would see them welcome the son and his partner into the family.”
"But he that shall scandalize one of these little ones that believe in me, it were better for him that a millstone should be hanged about his neck, and that he should be drowned in the depth of the sea."
But if they are "welcoming" in the sense of saying "this sex-themed relationship is acceptable," they are in the wrong.
I think we agree on that.
If they were offering them a bed for overnight, now that would be sin-sponsoring and shockingly wrong. Just as if their son were a divorced/remarried man accompanied by his "new" wife. No decent person should do that.