In relationships, always. https://philosophy.inhahe.com/2018/06/21/cheating-fidelity-loyalty/ The very fact that people call sexual freedom "cheating" is a travesty.
In a non-relationship context, cheating is okay in grade school because grade school is a kind of emotional and mental abuse forced upon children, the child cheating on a test doesn't actually hurt anybody, and it somewhat mitigates the abuse of having to memorize a bunch of crap and be physically or emotionally punished by their parents if they don't.
In a game, it's okay to cheat if all parties involved know you're cheating. If they feel offended by the cheating, too bad, it's just a game. The worse that can happen is the game becomes less fun, which isn't really a moral issue. Or also people's egos could be agitated, but that's purely a self-centered reaction and nothing that needs to be supported or protected. Besides, if everyone knows they're cheating then they didn't really beat you anyway.
If a person cheats in a game secretly, that's immoral because it's lying. It's lying about the ability of the person in question. And believing any untruth has a detrimental holistic effect on one's entire episteme and their ability to determine future truths.
If a person cheats in a situation where money is involved, that's not just. Money is like the fruits of misery/labor (and sometimes luck) and the power to purchase happiness, health or security, and if someone cheats then that's violation of whatever kind of financial-exchange agreement we're talking about. It can cause harm to someone in a way that they wouldn't have chosen if they weren't lied or if certain very relevant information weren't withheld. Actually, I take that back. It's perfectly fine to cheat against large corporations, because they're exploitative, evil, and have more money than they need anyway.
In a court case, cheating by the defense is okay if the defendant is actually innocent, and cheating by the prosecution is okay if the defendant is actually guilty. The cheating in this case is simply reversing a potential injustice. There's no point in going through the process except to effect justice, so there's nothing wrong with violating some rules of the process if it's a means to that end. If you don't believe in punishment in general (as I somewhat tend toward), or punishment in the particular case, or think it's excessive, then cheating by the defense is okay and cheating by the prosecution is not.
In voting (of all types), I'd say it's okay to cheat if it increases the number of votes for the option that has the better overall outcome for all, but the problem is you can't really advocate that because everyone has their own ideas of what's better for the overall outcome of all. You'd just end up with the people with the most conviction cheating the most (cough republicans), and we know that there's a negative correlation between self-confidence and competence, and that the wise man knows that he knows nothing.
Of course, I'm an exception to the above rules; I'm always right and I know it, so if I knew the details of a particular voting event and felt confident to determine what the best outcome would be, then I would say that the correct answer to when it's okay to cheat is that it's okay for the side voting for whichever option I say is the propitious option to cheat and not for the other side to cheat. For example, it's okay for Democrats to cheat when voting but not for Republicans. On the other hand, nobody would listen to me anyway; they'd all believe their side is the correct one, and they'd either cheat or not depending on their own morals, their ability to cheat and their chances of getting caught. Also I'd be vilified. =P
In the scenarios where I said it's okay to (secretly) cheat, it's still true that the misinformation can have a holistic effect on the psyches of the people who are fooled, but I believe that effect is less important than the positive effects of cheating.
