There wasn't any ground rules discussed as such. Yes she is supposed to tell him about her encounters but doesn't. After the Ryan incident She promised not to keep anything from the MC again but she still does. She knows that he sleeps around also but asks that he doesn't do anything with the girls from his work (depending on your play through) he does anyway, she has also said that she would like to watch him having sex with another girl.
This is where the game fails IMO. Even if the MC doesn't sleep around Anne will still sleep around with other's behind the MC's back and not tell him about ALL the encounters she has. I don't think it would matter if the couple did set ground rules from the start Anne would still be a slut and sleep with anything with a pulse, Young and old, I wouldn't put it past her to jump on a dog if it was about.
IMO this couple and the game are doomed. The only way I can see the dev saving this game is for Tony to confront Anne about her cheating, They have a heart to heart and Anne explains her thoughts and feelings when she goes off with other guys. IF they continue to be together they set ground rules that BOTH must abide by.
I'm of the mind that the problem in the relationship isn't Anne. It isn't possible to cheat in an open relationship. If it was, it wouldn't be an open relationship. What the problem happens to be is that for one reason or another Anne doesn't always want to disclose who she is seeing. Even though 9 times out of 10 her husband wouldn't mind and in fact would enjoy hearing about it (in most playthroughs). That 10th time it is because she is with someone who is inappropriate (MC's Father) or too attached (pitchfork angry cheating husband). Yet it is clear from other times she does disclose that she is emotionally involved with her partners, not just sexually, and isn't ashamed to admit how she feels in these instances.
So what is happening in the relationship with MC that she feels that she doesn't have to share sometimes? It isn't fear of repercussions and she doesn't need to worry about his feelings. He isn't going to get upset or tell other people what she has done. So the secrecy is pointless. Unless... she is feeling insecure in the relationship and is starting to feel that her husband doesn't care about her anymore. She does get upset one night about her husband being too eager to her about her having sex with someone else, professing that it makes her feel used. This is, of course, the night when the MC can choose to sleep with her mother. It is a bit of a tempest in a teacup, and it might have been as a result of an unpleasant sexual encounter that we are not privy to. Given the history of the couple to this point, it is likely that her anger was misdirected from the man she was actually upset with. It is fully possible that some of the encounters she doesn't disclose and we do not observe are not consensual.
So something about the lifestyle is hurting Anne and making her feel insecure in her relationship with her husband. Maybe even unsafe. It is opening her to mistreatment and possibly outright abuse by other men and she is probably too ashamed to admit it when one of these encounters hurts her because it will interfere with her husband's enjoyment of his fetish. Of course, it is just a theory, and it would make the game a bit of a bummer, but there doesn't seem to be many logical reasons why Anne would "cheat" on her husband in an open relationship. Perhaps this is something that is a bit of a pattern for Anne. Perhaps in her promiscuous college days not all of her encounters were consensual, and instead of dealing with her feelings or her trauma, she piled on more promiscuity to bury them. Perhaps the same this is happening again, and she feels her husband is not protecting her but rather pushing her back into those situations.
In any case, the couple has a lot to talk about. Is their relationship salvageable? In the long road, probably not. But that doesn't mean that they have to go down in flames. Maybe they need more rules and maybe they need fewer of them. Maybe it is best for both of them if neither Anne nor Tony is obligated to report who they are seeing unless they feel it is important to share. Because neither of them has been fully forthcoming in any playthrough. We know that MC has concerns and reservations that he is not bringing up to her. It wouldn't surprise me to discover that Anne shares them but will not bring them up for the same reasons. It may be that they are more on the same page than we think. Whatever is the case, all that is required is that they come to an agreement and not make demands that the other partner finds unreasonable.