That just might be the definitive criterion in society for determining official familial relations and what the child is expected to feel.
It's not DNA (donors are not fathers, right?), it's not sex (conceiving via IVF with your husband's sperm still makes him the father, right?), it's not nurture (your stepfather who raised you from day one after your father died while your mother was still pregnant with you does not have to be your "real dad", right?)
It's how your mother feels and to whom she is loyal. She can claim when you're 18 that your real father, who'd knocked her up behind a bar and whose name she never knew, is your real father and should have been paying child support - if he could have been found.
Or she can claim that your father, who'd been married to her for years, but left her while she was pregnant for a younger, more beautiful woman, and whom you never met, is nothing more than a sperm donor, and your real father is the wonderful man she married afterwards who loved you like you were his own.
Or she might keep the memory of her late husband, who died while she was pregnant, alive by stories, pictures, mementos, and often tell you about your real father, who was such a lovely man - although you quite like her new husband and he's the only dad you've ever known.
And the child is expected to feel whatever the mother feels - DNA, nurture and sex notwithstanding, as we're not talking about rational arguments when we discuss who the real father is, we're talking about feelings - and if the child feels differently, that comes at the high price of betrayal and disloyalty.