I'm not into NTR, but experience doesn't make a woman inferior, like someone else's leftovers. We're all people, not goods. Thanks for challenging that view, so that it's (maybe) his instead of part of a silent consensus on this comment board.
An extremely charitable reading of
@EdenSphinx's comment, however, presents a good point: wanting to sleep with your girlfriend an hour after she cheats on you does reinforce his image as lacking self-esteem. The author has given multiple clues that he has an unhealthy self-image, including getting off watching his GF cheat. This isn't a typical manga where being a beta is the source of his problems, however, or even a clear liability.
This is the odd NTR manga where being a beta might be necessary for MC's survival and happiness. Middle aged man (maybe an alpha) got the praying mantis treatment. An alpha MC might have dropped the girlfriend who wouldn't put out (end of this story), or might have pushed and gotten killed, while beta MC is surprisingly aware of the circumstance he is in and is flexible enough to navigate it so far. He's also not a player MC, trying to prostitute his GF so he can watch from a safe distance because of an NTR fetish.
These traits, like listening to his GF, make him a more sympathetic character. Spying is a betrayal that cuts him down, but it turned out to be important for his survival. Somehow, despite both of their betrayals, they are still in love too, which humanizes the GF and adds interest going forward. It's a refreshing change. Oddly, I want to see them become a happy couple, hopefully before she gets caught.
Despite all the lewd scenes, however, I haven't found this one particularly erotic, so maybe their particularities will hurt its popularity. I hope author develops and grows these characters, instead of milking their problems to sell more episodes like most romcoms do. If so, we'll see them grow instead of seeing snuff porn until the ax falls (maybe with the police showing up out of the blue).