Pastor Jeff is the one who destroys George and Mary's marriage in Young Sheldon, not Pastor Rob . The Cooper parents' marriage has been in a gradual downward spiral for a while now due to the pair's inherent differences. Now, it may be taking a turn for the worse, but while Pastor Rob is easy to blame for Mary and George's Young Sheldon problems, Pastor Rob is actually more responsible for their issues.
Currently, The Big Bang Theory spinoff is focused on Georgie (Montana Jordan) after he and his girlfriend, Mandy (Emily Osment), became pregnant. This big development has impacted all members of the Cooper family, including George Sr. (Lance Barber) and Mary (Zoe Perry). Aside from them arguing about how to proceed with the situation, the crisis has also forced the couple to re-examine their relationship. It's a particularly tricky subject because their marriage started because of an unexpected pregnancy as well.
Other people's unsolicited opinions have made things harder for the Coopers. Mary's reliance on Pastor Rob (Dan Byrd) has been particularly frustrating for George. That being said, while it's easy to pin this on Rob, the blame for destroying the couple's marriage should be put on Pastor Jeff (Matt Hobby). In the Young Sheldon season 5 penultimate episode, Pastor Jeff decided to fire Mary from her job at the church to keep the congregation from talking about her and her family. He made it seem like he was doing it for the betterment of the Coopers, but in reality, he just didn't want to draw the ire of his community for his ties to Mary. For years, Mary sucked up to Pastor Jeff, effectively prioritizing his issues, whether it's his personal or church-related dilemmas, over George's own woes in Young Sheldon. Now that Pastor Jeff has betrayed her, Mary still doesn't realize that he has essentially used her this whole time, which makes it difficult for her to stand up for herself — and this is something that irritates George.
Disappointed with Pastor Jeff's actions, Mary continues to ruminate about his decision in the hopes that it will make sense to her instead of simply accepting that he has turned his back on her. This way of thinking is keeping her hopes up that eventually she will be rehired at the church. But that seems unlikely to happen. As seen in the Young Sheldon season 5 trailer, George is reminding her that with their house still not being fully paid off and the bills piling up, it's important that she moves on and find a job elsewhere. This further strains the pair's relationship. George has always been worried about money; it's the one thing that continuously stresses him out. He started moonlighting at Dale's (Craig T. Nelson) shop, and considering how much work he has taken on in the last few months to make sure that their family stays on top of their financial obligations, it's understandable why he would be more frustrated at Mary for still holding out hope that Pastor Jeff and the congregation will bring her back.
Mary's refusal to let go of her ties to the church and prioritize her family is also the reason for her continued association with Pastor Rob since he's her former co-worker and confidante. They technically haven't done anything wrong, but they're both aware that there's a mutual attraction between them. Hopefully, Mary starts focusing on what she needs to do given her circumstances in Young Sheldon, and the only way to do that is to accept that Pastor Jeff has essentially abandoned her.