Pretty sure you don't have to tell or whisper to your agent about her, it's none of his business. If you don't sleep with her, you can lie to him and say you did. Also isn't your agents SO related to the girlfriend? Why didn't she say anything about it, she just blows it off as him being him ie. normal. And iirc you can choose your "sex" style no matter which path you take(don't recall exactly, haven't played it for a while). I'm also curious how old you are based on your comment about age. I'm 57 so I think I have a problem with your comment J/K... I chose not to tell him anything, and I didn't stay with the "GF" after dinner and sex, didn't do dishes either, cause that just ain't me. I believe those choices are available even on the "good guy" path. I'd also mention that you don't have to have sex with other women, you chose to. If your going to have guilt about it that guilt should have stopped you before you did it. If that's the case you have no moral compass and it wouldn't bother you anyway. There are games available where you can get cucked and feel guilty all you want, just say'n...
I don't really have the energy to address all of this, but I will say that I'm not personally feeling guilty, why would I feel guilty about exploring different button clicked options in a VN. I'm saying the character should perhaps feel guilty, which is a fair bit different. And I don't see how feeling guilt about cheating has anything to do at all with cuckolding, I'm not interested in cuckolding, I'm interested in exploring moral dilemmas. If you don't see how somebody could make a decision and then feel guilty about it you must have led a pretty care free life. Sometimes somebody knows something is wrong, does it anyway, and regrets it afterwards. This applies to many things in life.
I'm only in my late 20's, so perhaps you're offended on the age thing, but I've got many friends who are significantly older than I am, pretty much all of them are in committed relationships, the ones who are not are generally straight up bachelors. It doesn't really matter anyway though, open relations in general are a small % of overall relationships, obviously people commenting on a porn site are going to be somewhat more open to them on average and therefore somewhat more likely to engage in them, but they are not by any means the norm, humans being naturally jealous and insecure.
Its the last I'll say on it anyway, at least with anyone but the dev, if he says it's an open relationship then that's fine (although that should be clarified in game, I doubt from a narrative perspective that would have gone unsaid if it were the case), otherwise I think it's a narrative flaw, it's as simple as that.