[danaris]

Why are the only options "married couple" or "unmarried couple"? Or even "married couple" or "single parent"?

Why can't broader family structures be considered, and be stable?

[anamax]

Those are the options that have significant data.

No one knows how other models work. However, note that "stable" comes from the data. Maybe other models are stable, maybe they're not. Without data, it's simply unknown.