It's questions like these which emphasize the importance of communication in relationships. If you are in a relationship with someone, it is important to be in constant communication with each other and to come to agreements and compromises. With that being said, one should talk to their partner with what constitutes "cheating."
It may be awkward, but if you are in a serious relationship with someone, you would love them enough to muster up the courage to bring up the topic to maintain a healthy relationship.
Anyways, couples should talk to each other and make sure they are both in understanding of each other and so that both sides feel that it is fair. Having one person in the relationship being overly controlling and having their partner not even associate with particular genders would be an issue. On the flip side, having someone in the relationship being a pimp and not giving a **** would also pose an issue. So communication is essential to make sure neither of extremes occur.
With communication, both people in the relationship can discuss what IS and what IS not cheating. Not for other people, but for themselves. Is kissing someone on the cheek cheating? I don't know, discuss it with your partner. Is sexting with someone else cheating? I don't know discuss it with your partner. Is cam-to-cam cybersex cheating? Perhaps your partner would be able to compromise to allow it as long as it's a different person every time (to assure that you aren't really in another relationship.)
So what is cheating? Cheating is what you and your partner agreed to. What if you guys haven't had that discussion? Then cheating is what your partner thinks cheating is and you ain't gonna know what that is until you get yourself screwed.
If you have reasoning for why you want to continue doing certain things (like sexting) for example, then come up with a valid argument for your partner. We all understand that we have sexual "needs" to fulfill and some people fulfill them differently than others. The only this is an issue is because our society (and many others) puts an extremely strong link between sexual acts and loving relationships. But if both you and your partner discuss this topic with open minds and understanding, perhaps you can reach an agreement, a compromise, or perhaps your partner might have a suggestion that makes you both happy
I'm going say this one more time for emphasis: communication.