I agree, I don't think Kathryn is just going to start cheating on him, because she's bored, horny or any of the other usual motives for doing it. It will be because Luke has broken the 'contract' the two of them have agreed on, where they discuss everything together and make joint decisions.
With the sex drugs part of the story, the Dev has given himself a pathway, for Kathryn to eventually cheat on Luke. It might not be intentional, if someone gives one to her, without her knowledge. But maybe she would still view it that way, and perhaps wouldn't tell Luke about it, because she was ashamed or embarrassed, so technically it became cheating. The other possibility, is that it happens because Luke is cheating on her, and this is the consequence. She finds out about his cheating, gets upset and in this moment of weakness takes the drug, and that opens her up to future netorare content, rather than the netorase you get if Luke remains faithful. Luke's cheating must have a bad consequence for him eventually, because otherwise the Dev wouldn't keep suggesting that maybe he shouldn't do this?