Dating two people at once without telling is not exactly cheating, unless the people involved believe you are being monogamous. If someone does, then it is cheating if you do not correct this belief.
There is a gray area in casual dating that some people take advantage of — on the one hand, casual dating is not a formal commitment and one can say “But I never SAID I was being monogamous.” On the other hand, many people assume monogamy once the interactions get to a certain point. In a situation that’s gone on for six months, many people would assume monogamy if you never mentioned other lovers.
The ethical thing to do in this situation, IMO, is to say “By the way, I reserve the right to date other people unless we agree otherwise.”
How would your girlfriends feel if they knew you were seeing and sleeping with other women?
I believe if you know something that would matter to a friend or lover, and you are not bound by a promise not to tell, it’s ethically appropriate to tell, and inappropriate not to tell.
Another issue (non-ethical) is intimacy — certain kinds of intimacy are not possible if you keep big secrets from someone. However, you may not value the kind of intimacy that comes from sharing everything you are, and that’s OK.
A third issue concerns practicality: if your girlfriends believe you’re being monogamous and they find out otherwise, they are likely to feel betrayed and that would make your life difficult. You might lose one or both of them. If you don’t care whether you lose them, then of course this is not your concern, but it’s something to think about.
If the part that you enjoy is leading two separate lives at once, then consider that it is possible to do so without lying by omission. You just let the people that you date know that you are or may be dating other people and that you are not willing to give any further details about your dating. That is ethical because it allows them to make their own decisions concerning their involvement with you, knowing that there are things they
may not know about you.
It’s not cheating unless you’re caught? Sorry, but that’s not any
definition of cheating I agree with. To me, if you have an agreement or an
understanding, and you don’t abide by it, that’s cheating, whether you get
caught or not.
Not disclosing parts of your lives is fine as long as you *agree* to do so.
But if you do it without agreeing, and there is a likelihood that someone
thinks you’re being monogamous, then it’s cheating, and it’s unethical, IMO.