Really? I got the impression that it's leaning more towards a reality check about trust and their relationship in general, maybe reevaluating whether they should continue the play or not.
Sure, it's possible that Hana is now fucking Yuma behind Tatsuki's back in the cliffhanger, but it would feel very forced and there have been fake-outs like this before, Tatsuki thinking that "there's only one thing that can be happening" while it actually being something innocuous would be very in character for him. Moreover, he was just going to quickly buy some ingredients Hana needed and it would be very risky to count on him staying away long enough for them to do anything if she actually wanted to cheat on him for real.
My guess is that either 1. he misinterpreting her "moan" and they are doing something else, for example reaching for an item they accidentally dropped under furniture in a difficult position ("that's the place") or 2. it's Hana and Yuma purposefully testing if Tatsuki trusts them alone or if he rushes home and tries to sneak up on them again. Option 1. would probably make Hana sad again about his lack of trust if Tatsuki rushes into the room like "STOP HAVING SEX" and they're not doing anything, this might most likely end with them not continuing the play for a while if ever. Dunno how option 2. could go if Tatsuki has bought everything he was supposed to and he wasn't especially rushing back, but we'll have to wait and see. In any case Fallen doesn't seem to have intended true netorare for this game at any point, based on his postings, so I'll trust him until he shows he has changed his mind.
Anyway, we need a hero.