I'm still waiting for you to explain the rambam.
Do you at least recognize some contradiction between halacha 1 and 16? How do YOU explain it?
I'm happy to inform you that indeed this is considered a major stirah in rambam. (And shu).
The Beis Shmuel asked this kasheh in even haezer 1:1. The taz provides a dochuk teretz that the first rambam is talking about a situation where the man has two wives...
The kashe is stronger because the first ramba m is based on the mishneh yevamos 61, and the second one paskens like R Yehoshua of laerev on 62b. The gemara says that R Yeshoshua is cholek on the mishna!