The "Have Sex All Together" button disappears from PC Mom's menu after I reach 32/38 relationship. This happens regardless of the PC's gender and femininity, regardless of which parent initiates the first Sex All Together scene. Also, after unlocking the repeatable Have Sex All Together scene, the button appears twice in PC Mom's menu: next to the Chat button, and next to the Have Especially Risky Sex button, while the father only has one button like usual. Neither of PC Mom's Have Sex All Together button returns after passing any of the mother's or the father's relationship benchmarks.
I'm using a save from a game version before 1.2.6 on v1.2.7a, and I used the computer's debug option to reset both parents' relationships