Young Sheldon Season 5 hints at why Mary never talked about George's affair in The Big Bang Theory

The recent episode of Young Sheldon gave an insight into why Mary Cooper never mentioned her husband George's extramarital affair in The Big Bang Theory. But she was often seen ranting to Sheldon about how his father was not responsible enough. Every time Mary (played by Laurie Metcalf in The Big Bang Theory) visited Sheldon and Leonard in Pasadena, she never missed a chance to talk ill of him even after years of his death. 

Interestingly, Sheldon (Jim Parsons) had revealed to Penny (Kaley Cuoco) the reason behind the three knocks at someone's door is because he walked into his father having a sexual relationship with another woman when he was 13 years old. But we never heard Mary talk or discuss it on the parent show The Big Bang Theory. Both Sheldon and Mary often talked about how negligent and lazy George Cooper Sr (Lance Barber) was. 

Ever since the revelations, fans of the show have been eager to find how was it that George had an affair with and its aftermath. In Young Sheldon, George has been portrayed as a decent family man until now. In the new season, however, George was seen spending time with their neighbour and recently single mother Brenda and feeling guilty lying about it to Connie (Annie Potts) and Mary. George and Brenda had also decided that they would not walk into the bar at the same time, at least for a while. But it has not been revealed so far if Brenda is the one he cheated Mary with. 

We believe all the theories will be put to rest in Young Sheldon. The reason why Mary probably did not talk about George's affair in The Big Bang Theory could be because of her guilt. In the last episode titled 'Money Laundering and a Cascade of Hormones', Mary is seen checking out her young colleague Youth Pastor Rob (Dan Byrd), whom she despises. She even had a 'hot' dream of making out with him. Her attraction towards this younger lad is evident when she meets him the next day and is unable to speak or even look at him. Her feelings might not just go away that easy as she also works with him. She of course feels guilty and struggles to stop herself. Being religious and God-fearing, fantasising about another person makes her feel very guilty. 

If the new plotline develops further and if they have an affair is yet to be explored in the series. But it was neither talked about in TBBT. There was no mention of her cheating while she was married to George or probably, Sheldon himself wasn't aware of it. If she had an affair with the youth pastor, say even before George does, it would make it quite clear that this is the reason why Mary never spoke about her husband's extramarital affair in TBBT.