True. But this does not apply to the marriages of Charles IX and Henry III of France...both of whom died without surviving issue...I heard it. Don't know enough about the Valois to say one way or another. But it is the kind of subtle answer that 'feels right' that historians wouldn't pick up on right away and I've encountered before.
It's also the kind of thing those involved wouldn't nessacarilly understand, in the realm of "why does sex with you seem really weird sort of way."