Why else would Michael fixate on a boy? And, why would Kay, when informing him about her abortion, shout out "It was a boy, Michael!...and I had it killed because this Sicilian thing must stop!."
I always thought the Sicilian "thing" sounded forced, unnatural, or whatever. Why didn't they just say what the thing is?
As I'm sure most here recall, Coppola and the surviving stars did a panel at the Tribeca Film Festival a couple of years ago. During that discussion, Coppola revealed that the abortion angle was Talia Shire's idea (around 1:10:00 here):
https://youtu.be/Xd71uVNlbLsThat it's an outsider's insertion to the script may explain some of the questions it raises, like, "Who the heck is going to abort Michael Corleone's child?" But it also explains why you then work your way backward through the script to find Michael dancing and asking Kay if it's a boy; and then freaking out at Tom over whether the miscarried child is a boy. They're just building an emotional ladder that Kay can eventually pull out from under Michael, whether it's plausible or not.
