There's also asexuality. Love your partner but don't wanna fuck 'em? Get a divorce you deviant! Because apparently sex is required for a happy marriage and if you don't have sex because you're not interested in it, then you're obviously a pervert or a prude who deserves to be unloved.
Unless you're both asexual, or are open to the sexual one fucking around,
Yeah, but then you get back to the person in OP's post telling people to get a divorce regardless. My point is that you can have a happy marriage and be poly and/or ace.
Edit: Also, I'd think that issue would come up long before marriage and would likely be solved by that point.
Ironically this is one of those things that's easier to deal with in a poly context - your partner isn't your one and only so if they're ace and you're not, you're allowed to get those needs met elsewhere and still have a loving romantic relationship with them.
One would hope that you'd solve issues like that before getting married. Whether that means an open marriage, having a partner who's also ace, finding ways to fulfill your partners needs without having sex yourself, etc.
You're not wrong, but I think most married couples that involve someone who's ace would have that problem solved by the time they get to marriage.