Okay I think we all need to admit this fact; Tsuki is now actively thirsting over her daughter's boyfriend, and that's a major issue.
You can't pull the whole "Oh those mental scenarios of Sakurai trying to seduce Tsuki in are just Tsuki's overactive imagination getting the better of her. She's not sexually attracted to Sakurai or would ever consider cheating on Fujio." excuse that's been going around since this sub plot was introduced.
Tsuki's reaction to seeing Sakurai naked mirrors Hana's reaction, being equally scared and aroused. PLUS, this chapter contrasts to how she first saw her future husband naked for the first time in chapter 99, where she seemed more creeped out to the point where she passed out from the shock. This chapter, taken with all the other moments of Tsuki's "worry" over Sakurai pursuing her, shows that what Tsuki is feeling is not just delusions of paranoia, but a repressed desire to want to start something with Sakurai.
The point is this chapter crossed a line. The NTR bait "joke" ran its course a long time ago. There's no wiggle room to justify or downplay what is going on, no matter how some of you here may try to. If this was real life, or even another rom-com manga, you would call out these kinds of fantasies for the borderline infidelity it is, no matter the gender of the person having said fantasies.
It would be one thing if Tsuki was a widow/divorced or even if Fujio was a bad husband (although the latter case doesn't justify physical/emotional/mental cheating), but neither of those things are the case. This whole NTR bait subplot needs to be nipped in the bud.
And I know people here are going to say "Oh it's a rom-com manga with sex jokes. This subplot doesn't mean anything. Don't take it seriously." And here's the thing; I would normally not take a subplot like this seriously if the manga itself didn't take its central romance seriously.
But the manga does take the main romance very seriously. It wants you to root for Sakurai and Hana to be together and work it out. Why else did we spend an entire arc for Sakurai to learn how to not cuddle people while he sleeps so Hana can have peace of mind? Why else did we bother with 130+ chapters of build-up and character development? It even wants us to take the side romances seriously like with Kiri, or Ami, etc.
You know that if Hana (or Fujio) were to learn about what happened this chapter, this wouldn't be dismissed with a shrug and a punchline.
This subplot with Tsuki and Sakurai shouldn't be dismissed as a harmless running joke, or even NTR doujin bait that doesn't really matter. It needs to be called out and criticized.
Bottom line is the subplot needs to be fully resolved, or at the very least needs to be put out in the open so Hana and Fujio aren't in the dark on this (you would want your significant other to be open about something like this). This subplot needs to be retired, yesterday.