Even if there were no restrictions, why in the fuck would Sage want to have a threesome with her mom and vice versa? We already saw how Jade reacted if you fucked both of them, and Sage goes apeshit over Chad dumping her, not to mention the cheating. Just imagine Sage learning about how the first guy she's ever loved is also banging her mom, and asked for a threesome.
And speaking of coomer mode, I think that is a reason why Jade is barren. For one, it means her pussy wasn't "ruined" by having two kids.
Two, it means you've been somewhere Tybalt hasn't
Three, to the NTR crowd, it would be unmistakable proof of Stephen violating OUR milf. Sure, they've been married for twenty years, and Jade tells a story of how she let Stephen drunkenly fuck her, and how his dick isn't big enough to fuck her in certain positions, making it obvious that they've had sex in the past. BUT! If you stick your fingers in your ears and say LALALALALA CAN'T HEAR YOU loud enough, add the fact they're stuck in a loveless marriage and Stephen is busy with college chicks, you can easily pretend she is in pristine condition. Or close enough.