Sheldon's (Jim Parsons) experience in may be the reason why he regularly criticized Penny (Kaley Cuoco) for her relationship choices in . The socially-inept genius continues his story through the CBS spin-off that tackles a portion of his childhood in Texas. Given this, several aspects of making fans understand his behavior more in The Big Bang Theory. That potentially includes his strong opposition to Penny's adventurous sex life.
As original cast members of The Big Bang Theory, Parsons and Cuoco played their respective roles for 12 seasons until the sitcom wrapped up in 2019. Sheldon and Penny arguably had the most interesting dynamic in the show. The pair didn't have any romantic inkling towards one another; Sheldon was the only male cast who didn't attempt to romance Penny, fully understanding that Leonard (Johnny Galecki) was smitten by her. Instead, what they had was a genuine friendship; there’s even an argument that Sheldon may be closer to her than his best friend, Leonard. But their relationship also had a dark side, with Sheldon typically ragging on Penny's promiscuity.
This was the main criticism towards their dynamic in The Big Bang Theory. But Sheldon's disdain for may have something to do with his trauma from childhood. It's been established that Sheldon's dad, George Cooper (Lance Barber) had many affairs. At one point, the socially-inept genius even accidentally caught him having sexual relations with another woman. Penny sleeping around, albeit totally consensual, may have reminded him of his dad's infidelity and how that negatively affected their family. Now, Young Sheldon has yet to tackle this particular plotline from Sheldon's childhood, but the prequel is already laying the groundwork for this rather dark narrative with George's growing attraction with their newly-single neighbor, Brenda Sparks (Melissa Peterman). It's uncertain if there's anything that would come out from their obvious crush on each other, but it establishes the idea that the Cooper patriarch has the tendency to have romantic/sexual feelings for someone other than his wife.
Interestingly, of catching his dad in the act of lovemaking with a different woman with Penny — something he never shared with anyone else. In The Big Bang Theory season 10, he decided to open up and explain why he developed the annoying habit of knocking thrice before entering a room. Apparently, after he accidentally barged in on his dad doing the deed, he figured knocking excessively would give someone enough time to make themselves presentable to prevent another awkward moment. While the original sitcom presented this as Sheldon simply telling a story, this could also be his way of secretly making Penny understand why he had been so opposed to her sleeping around. Granted that this wouldn't really make up for all of the years of , Young Sheldon confirming this idea could be an indication that CBS is acknowledging its sexist depiction of Cuoco's character in The Big Bang Theory for the sake of comedy.
Young Sheldon is often criticized for contradicting the established canon from The Big Bang Theory. But it seems to be trying to make up for that in recent years, with the show actively building on some of the story elements from its parent series. In season 5 alone, it has explained why Sheldon hates engineers and why he uses the word "coitus" instead of "sex." So, it's not outside the realm of possibility that Young Sheldon also makes up for some of the original sitcom's problematic tropes like Sheldon's inappropriate treatment of Penny.