I believe you missed the point, Kat's purpose was never about sex or anything meaningful. The role Kat plays in Phil's life is something he can point to as evidence that he's not any of the following;
- An Incel
- NEET
- Autistic
- A child
- A nerd
How many times have you heard "I'm a grown adult, dood! I have a wife!" That's her purpose. Evidence that Phil is an adult that has an adult life and adult relationships. That's it. Phil burying his gout sausage into a live coochie was never the goal. Some people guess it happens once a month, but I seriously doubt him and Kat ever have sex.
But there's plenty more discussion of Kat and her role in Phil's psychology in the other thread