Let me add a touch of armchair psychoanalysis to this: Ma’ Burnell wore the pants in the house while Phil was growing up.
Phil had a superficial relationship with his father, which has screwed up how he sees women. He’s a closet asexual, who thanks to his childhood sees women as mothers instead of as objects of sexual desire.
Phil “needs” a wife because in his autistic brain, “mature grownups” are married and have “families”.
Phil married Khat to fulfill his internal self image of a “mature, successful grownup”.
Alas, he’s incapable of seeeing women as anything but motherly figures.
Khat is pretty screwed up herself, so she’s either fine with it, gets some potent dick on the side, or is biding her time, enjoying the financial security until a better option, one that actually makes her ovaries tingle, comes along.