Cause the Abraham-Sarah-Hagar menage à trois worked out so well
I could see Katherine arguing against it that if God saw fit to bless Sarah with a legitimate child when she was in her 90s then why couldn't He do the same with her? Not saying she will, but it IS a counter argument Katherine COULD make.
That said, didn't Henry petition the pope to allow him to do just this saying "well, if Solomon had five hundred wives, why can't Henry of England have two?"
I know the pope offered to recognize any child of Henry and the Lady Boleyn as legitimate (even if born out of wedlock) rather than grant the annulment, so his Holiness was perhaps also thinking of a sort of Abraham-Hagar scenario.
@FalconHonour: (re: derailing) no worries. Gives me some new fanfiction to read
Agreed with you on Ms. Dormer as Anne, Claire Foy should stick to playing Elizabeth II
@desmirelle: while we're going all Old Testament, there is a case in the Pentateuch though (Numbers 27) where a man has five daughters but no son. The daughters were allowed to inherit (by God's command no less), originally with no restrictions. It was only when the leader of Manasseh raised a fuss that the girls were wed to their cousins (again at Divine order).
The question is simply who gets to marry Mary. One of Arthur, Viscount Lisle's daughters (presumably Frances b.1519) could be born male. The elder earl of Lincoln (b.1516) could survive, or the Marquess of Exeter could have a son with his first wife (the Viscountess Lisle) before she dies in 1519.
Arthur Plantagenet's kid would be beholden to Henry for nearly everything, since he would be heir to almost nothing. Lincoln is a nice option (but Katherine and others (Norfolk) might object to his Brandon blood). Courtenay/Exeter could likewise make for an interesting option.