There are three persons, A, B and C. A and B are together. C and B are friends. C has casual sex with A. It is obvious for both A and C that they are not going to start a relation. They have casual sex because they feel like doing it. Is C doing something wrong towards B?
Well, either A and B have an open relation, or they don’t. Only the second case matters here. B wrongly assumes that A’s having sex with another person endangers the relation A and B have. C is pretty sure of the contrary (for this particular case, at least). Should C refrain from having (casual) sex with A? Why?
It could be said that C must do something bad to B, since they are friends. From C’s point of view, the relation between B and A is at no point endangered by C’s having sex with A. Of course, B might feel threatened, but we are not responsible for the irrational feelings our friends might have.
The only thing left is this. B trusts C not to have sex with A. By having sex with A and not telling anything about it to B, C appears to be cheating. However. C could tell B: “I have never promised you that I will not have sex with A. Since you seem to be irrational about such matters, I am choosing not to disclose anything of this kind to you. That is, if I had (non-threatening) sex with anyone being in a relationship, I would not tell you about it, since you would (wrongly) judge me”.
So C might simply be avoiding to talk about a matter B seems to be irrational about. Suppose C was a theist and B an atheist. One could have reasons to be an atheist, but B, in this imaginary case, hasn’t any reasons. Is C under an obligation to talk about going to church with B? Of course not. Suppose the church was in B’s hose, without B’s knowledge. However, things were such that B’s life and atheist values could not be disrupted by this fact (the church being at B’s house). Should C tell B about it? Not really.
It could be said that A has some obligations towards B, since they are in a relation. Does C interfere in any way with these? Even if A’s staying in good health is part of the obligations A has towards B, as long as C knows that making sex with A does not endangers A’health, everything is ok. No other obligations can affect how A’s body is used by A.
Bottom line is this. If you are irrational in your jealousy, a (healthy) friend which sleeps with your partner without actually putting your relationship in any danger is not cheating on you. Also, as you might have noticed, gender does not matter here.