The rules are about what constitutes infidelity are different for everyone, and you probably shouldn’t let me or anyone else give you a pre-made prescription for setting comfortable boundaries and expectations in your relationship. It’s your thing, and so please feel utterly free to disregard traditional notions of what you “should” or “shouldn’t” do based where your relationship is according to a very limiting set of tired labels. Every couple has to hammer out what limits feel right for the two specific, unique humans who are existing in this relationship that is not like any other relationship that has existed before.
That said, there are some scenarios that enough of us can identify with and have experienced, that we can make a general ruling as to whether or not they should be allowed.
Emotional affairs
Maybe cheating. It depends if you’re sincerely having a legit emotional affair – you’re in love, you act in love, you talk about being together, you lean on each other and confide in each other to the exclusion of your “official” partner – or if you’re just being accused of having an emotional affair by someone who is insecure and is having an overreaction to you simply having other friends.
Secret friendships with persons of your sexually preferred gender
Not cheating unless, again, it fits the criteria of an emotional affair, or obviously if you’re grinding on each other on the regular. No healthy, sane person can possibly get everything they need from just one person. Two people in a romantic partnership who don’t have any other important relationships in their lives baffle me endlessly; are you weird human anomalies who are magically fulfilled with just each other, or are you kidding yourselves and going without fulfillment because one or both of you is too scared to to acknowledge that each other isn’t enough and allow each other the freedom and trust to seek out other people as friends?
Looking at Tinder
Tinder scares me a little, but I can definitely see the appeal of killing time on an app where you can get an infusion of consequence-free superficial validation of your hotness. If your boo is on Tinder just to judge people for funsies, or to collect compliments but aren’t actually meeting people from the app, they’re not cheating. They do need better hobbies, however.
Having an online dating profile
Having? Not necessarily cheating. Although it is a cute gesture to delete (or let’s be real, deactivate. It’s not like you’re getting married.) your profile when you decide to be exclusive with someone. But actually using your account? Yes, that’s probably cheating. Actively using your online dating profile to talk to people with romantic/sexual intentions while in a committed relationship is cheating. I don’t think that merely talking/texting/emailing with someone is inherently cheating, but the fact that it’s happening on a dating site sort of clarifies the intention right away. Like, we all need friends outside of our relationships. You don’t need to find those friends on a dating site. That’s suspicious to the point of being basically unbelievable.
Kissing someone once
Chemistry is a hell of a thing. Sometimes, even with the purest of intentions, you get swept up in a moment with someone and even if you’re consciously trying to not cross the line, you occasionally have to drop a toe over the line before you even see it. Verdict: if you back it up, and come clean to your significant other about what happened, you’re probably in the clear. Moments of fantastic, rare, exciting connection with another person are the glittery exclamation points of life. They’re what makes the world turn. No one should have to go through life without having those heart-stopping “wow, this person!” moments, and that remains true even after you’re in a relationship. That said, when you’re single and you meet someone remarkable, it can be automatic to take the encounter to a physical place, even just to try it out. Every awesome person is your potential soul mate. But when you’re no longer on the market, and still don’t want to give up meeting special people, it can take a bit of adjustment to figure out how to do that without, uh, making out all over their perfect faces. It’s not flawless. It didn’t wake up like this. But you’ll figure it out. Hopefully you were lucky/smart enough to end up with a partner you can openly talk about this stuff with.
Side note: Figuring out how to be faithful in a long-term way can feel unnatural and difficult for a lot of people. As long as your person is being honest and is earnestly wanting to not be a shitty, horrible cheater, try to be secure enough to be compassionate. It’s a journey and you’re on it together. Don’t be each other’s judges; be each other’s teammate.
Anything that happens while you’re blackout drunk
No, dear rapists, this is not cheating. If you have sex with someone who is too drunk to know what the fuck is happening, you are raping that person and they are not cheating. That said, I would find someone who regularly gets blackout drunk just as undateable as someone who cheats. Not cute.
Anything that happens while you’re “on a break”
I hate to break it to all the Rachels of the world, but Ross was totally fine when he boned the girl at the copy place. Trust me, I get no pleasure from admitting that because as a person, fuck that guy, am I right? Regardless, being “on a break” means exactly that: you are taking a break from the relationship. Period. As with everything on this list, there is plenty of room for custom tailoring this rule; if you want the space to focus on work, or family stuff, or just to get some quiet time to do a little evaluation of yourself or your relationship or whatever but don’t want the complication of sleeping with other people and having to deal with that after your break, then say so. Lay it out there. You can’t get what you want – or avoid what you don’t want – if you don’t speak up. But in the absence of specifically designating that a “break” still requires keeping it in your pants, any break-time genital antics don’t count as cheating.
Vacation hook-ups
Do not be an asshole about this. Just because you’re in a different time zone or country. In fact, I’m gonna say this is one defense of cheating that I wouldn’t feel super eager to forgive; at least with other moments of infidelity, it was admittedly a mistake. Vacation Apologizers are douchey monsters who are looking for any excuse to set their sex parts loose. I think cheating is very often the kind of relationship hiccup that can be worked through and moved past, but anyone who would say something like “it doesn’t count because I was in Costa Rica, brah” can find his shit in a box to the left, to the left. 
