One of the new characters introduced in Young Sheldon season 5, Byrd's character started as a Sunday School Youth Pastor. His arrival almost caused a major problem after both Missy and Mary developed a crush on him. Neither has found out about their shared admiration of Pastor Rob, but while Missy seemed to have totally moved on from it, her mom has just gotten more infatuated with him. After dreaming of sleeping with her co-worker, Mary tried keeping her distance from Pastor Rob.
However, regularly working together made it difficult. Eventually, Mary and Pastor Rob developed a friendship, but it has become obvious that there's more to it than just a platonic relationship. Mary has started opening up to Pastor Rob, even prioritizing his thoughts and opinions more than her own husband. Although the pair, technically, hasn't done anything wrong, Mary's over-reliance on him is arguably an emotional affair — a more complicated type of cheating, but an act of infidelity nonetheless.
In The Big Bang Theory, Mary was painted as a no-nonsense devoted mother, but things are a little different in Young Sheldon. Although there is no doubt about her devotion to her kids, particularly Sheldon, she isn't the martyr matriarch she was initially painted to be. Young Sheldon features a deeply flawed Mary who has strong tendencies to be self-absorbed and self-righteous. When George opened up about being miserable in the Young Sheldon season 4 finale, Mary decided to mock her husband's woes, failing to recognize all his sacrifices to make her happy.
For some reason, Young Sheldon season 5 seems to have dialed things up when showcasing this side of Mary. She insisted on Georgie marrying Mandy (Emily Osment) not because it was the best for the couple and their baby but because she was afraid that having a grandchild out of wedlock would affect her image. More recently, Mary continued to deny that her growing closeness with Pastor Rob is bordering on inappropriate. In both instances, George pointed out these issues, but instead of re-assessing, Mary remained adamant that she wasn't doing anything wrong. In fact, she remains convinced that she is the victim in this situation because all her efforts are supposedly underappreciated.
After five seasons, Young Sheldon's version of George is still different from The Big Bang Theory's. While Young Sheldon seasons 6 and 7 are already confirmed, the CBS comedy is running out of time to show his transformation from a devoted father to a negligent one that Mary used to talk about in the geek-centric sitcom. That being said, there is another way to resolve these George inconsistencies without totally ruining Sheldon's dad in Young Sheldon. Even an emotional one, Mary's affair could explain why George got a bad image in The Big Bang Theory. The Cooper matriarch may have intentionally ragged on her husband to conceal her own misgivings in their marriage; that way, the focus would always be on George's flaws instead of hers.
It's worth noting that only Sheldon and Mary badmouthed George in The Big Bang Theory. Despite appearing in the original sitcom, grown-up Georgie and Missy in The Big Bang Theory never poked fun of their dad. Considering Sheldon's special bond with his mom, it's possible that he simply followed Mary's lead. Because Sheldon has poor social skills and is mostly unaware of what's happening in their Texas home, he may not notice Mary's flaws, including her borderline affair with Pastor Rob. Missy, however, appears to know what's going on after she caught Mary and Pastor Rob sharing a moment in Young Sheldon season 5 penultimate episode.
It's curious if she chooses to maintain contact with Pastor Rob, especially since George made it clear that he doesn't like her former co-worker. If she does, then it's possible that their growing closeness blossoms into a full-blown affair moving forward in Young Sheldon. If this happens, then it establishes a strong motive for Mary to destroy the image of George in The Big Bang Theory.