Much like with Jhyamsper Kittee, I seriously doubt Phil would give much of a fuck about Lil' Tyrone. He seems more likely to just say it's Khet's problem and goat laugh it away. It's not like he gives much of a fuck about romance, it's a loveless, business-like marriage where Kat ocasionally makes mashed potatoes and Phil gets to LARP as a mature adult. Kat's already backed out on being a LARPing implement, refusing to even provide Phil with creeper shots. If she wants a kid, she's gonna get a kid, Phil's opinion on the subject be fucked.
And if Phil wants to dispute raising the chocolate drug baby as his own, or at least splurge on them as if they were, Kat can just divorce him and fuck off to collect alimony. I've said this a lot, but Phil was a fucking fool to put a ring on it. Kat's got him by the balls, and she's probably not merciful when she has the upper hand. She cheated on Jeff. She won't even think twice about cheating on Phil if the opportunity arises and she feels like it. Chances are she's been fucking around for a couple years now; If your partner won't get you laid, and won't let you get laid, you'll just start to sneak around. It's a tale as old as time. Kat's twitch persona was a basic thot. She's most likely not as sexless as Puritan Phil.
As long as Phil can claim to be married to a potato autist, he'll keep Kat around. That's literally all he wants. Totally not an incel/unable to perform/deep in the closet, guys. Even if it comes with the normal beard move of just having a kid with someone else. There are plenty of beard marriages where the gay man just takes it in stride. Hell, a lot of them are legitimately good dads to the kid. Not Phil's case, of course, but the thought of him being forced into becoming an open wallet for Kat's Lil' Tyrone amuses me.