The Cooper matriarch has gone through a lot in the last several months. She unexpectedly developed a crush on his co-worker, Youth Pastor Rob, learned that Georgie is going to be a dad out of wedlock, and then got fired from church, made worse by being shunned by the congregation. She has been trying to get used to her new normal, and for what it's worth, there seem to be some upsides to her current situation. That includes rekindling the romance with George, although she may have been carried away by their revitalized relationship, as she suddenly has urges to have another kid.
While George and Mary were having their frisky nightcaps on Young Sheldon season 6, episode 5, Mary unexpectedly suggested that they have another kid. This shocked George, but his wife was adamant, explaining that it was the perfect time for them to have a fourth child because they have extra time on their hands. While this may have come out of the blue, Mary's urges may have to do something with wanting to feel young again, as she sees Mandy carrying and going through the same thing she did when she was pregnant with Georgie. Ultimately, this could be linked to her age insecurity with regard to her relationship with Pastor Rob. Earlier in Young Sheldon season 6, she accidentally bumped into him while on a date with a significantly younger woman, which made her insecure about being older.
When Pastor Jeff almost destroyed Mary and George's marriage by firing her, there was an assumption that it would also spell the end of her connection with Pastor Rob. After all, they only ever really get to see each other in church. However, their first and only interaction on Young Sheldon season 6 proves that somehow, Mary still harbored an affection toward her former co-worker. It's worth noting, however, that a lot has already happened since that encounter. Mary and George's marriage is on the mend, which is an unexpected element of The Big Bang Theory prequel's current run. It does seem like Sheldon's mother is genuinely enjoying rekindling the romance with her husband; so much so that even Missy finds it inappropriate and annoying. That being said, one can never be sure of Mary's feelings towards Pastor Rob until their paths finally cross again on Young Sheldon season 6.
For now, this frisky side of Mary that TBBT first showed is here to stay, and she will maintain this persona unless she inevitably returns to church. CBS doesn't appear to have any plans of doing that anytime soon, allowing Young Sheldon to explore other facets of her character. Just because she isn't going back to church yet doesn't mean that CBS can't explore her complicated relationship with Pastor Rob, however. Medford is a small town; there's always a chance of them running into each other. It isn't outside the realm of possibility that they foster a friendship outside their common attachment to religion. If anything, this is where they could finally give them the opportunity to act on their clear mutual attraction with each other.
Mary's appearances on The Big Bang Theory already confirmed that the Cooper matriarch will, unfortunately, not get her wish. Sheldon only had two siblings — Missy and Georgie, who both attended his wedding to Amy. While Young Sheldon is infamously known for its lax approach to preserving continuity, this isn't something that they can openly contradict, otherwise, the family comedy will totally break canon. It's in the same boat as Sheldon's story about George's infidelity and subsequent death.