I would strongly favor honesty in both those situations once the child is mature enough to understand, except, in the latter case, when it's clearly in the child's best interest not to have a relationship with her biological father.

> We also find it ethically acceptable for a couple not to ask who fathered their own child.

That sounds to me like an extremely toxic relationship.

> We also generally find it ethically acceptable to form and especially continue relationships based on factors other than true love, including stability of a family environment and economic stability.

Those are at least real emotions and real decisions made honestly, not elaborate and unthinkably cruel deceptions.

Imagine you learned the person you thought was your parent, or sibling, or spouse, was really another person entirely. They never loved you, they never wanted to spend time with you, and all your memories with them were a lie. They were just a hired actor the real part of your family paid $50 an hour to deceive you.

It's hard to imagine anything that could be more painful. It's far worse than a death in the family. It'd be on par with learning your loved one were a murderer or rapist. It's like a particularly unsettling Twilight Zone or Black Mirror plot—I'm aghast that it's a real thing.

"We also generally find it ethically acceptable to form and especially continue relationships based on factors other than true love, including stability of a family environment and economic stability" In this case, both of the people involved are grown adults and usually know reasonably well what's going on.

The real problem here is how deep a break of trust this would be if they ever found out, and how wrong of an idea this would give ppl of relationships.

Wait, we do? Social issues aside, what about family medical history!?