Infidelity has everything to do with that child because that is how they came about. Of course the blame is on the person who was unfaithful but when people see that child they see infidelity(side baby).
I'm sure they probably treat the adult worse than the child. I don't even agree with treating the child differently but I get it. Folks is hurt so they take it out on everybody they feel is a part of that hurt.
Only thing that doesn't make sense to me is family members having a problem with it who aren't tied to the spouse that was cheated on.
Meaning, if somebody's son has a kid outside of their marriage............there's no reason for them to treat the kid any differently because it's still their grandchild either way.
Spouse who got cheated on.........understandable.
Family of spouse who got cheated on.............understandable.
Half-siblings of the child who was the product of infidelity.........understandable.
Anybody else..................not understandable.