Another way to look at it (though I generally hate the condescending image of the third as the "child" of the couple they get involved with... ew... bear with me):
What would they do if they had a child out of wedlock? Raise her with the family, pretend she's just a foster child? Love her privately but keep a distance publicly? Let her appear to the rest of the world as an orphan? Go so far as to not even let her siblings know the truth? All in the name of respectability? Could they claim to love their child, yet treat in such a way, all the while telling her to look at the bright side: at least she has parents, even if no one else knows it, at least she has a loving home, even if it's full of lies? Is that love or self-interest? Could they expect such a child to come out undamaged? To not feel, on some level, no matter how often she is told privately that she is loved, that her existence is a shameful one?
Is this that different?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.