Well, for any breeding to work at all, creatures have to be very, very similar. Certainly not man and machine. The breeding rules here don't bother me very much, especially since I suspect they are artificial, set up by the Cylon god or final 5 to put restrictions on the 7 Cylons, probably to force them to breed with the colonials.

To give you a similar example, while a male horse can breed with a female horse and with a female donkey, he can't breed with a male horse, the one most like him.