The one and only rule of cheating is, are you hiding it? In other words, if you are in a relationship with person X and you meet person Y, if you do anything at all with person Y, whether it's a hug or sex, that you have to hide from person X, then it would be cheating. Some people are more open than others and it depends on the individual and the grounds of the relationship. If you know person X is uncomfortable with you having any kind of physical contact with anyone but them, and you do it and hide it, then you've cheated on them. If person X is fine with you getting blowjobs from other people, but nothing else, then anything else you do that you hide from them is cheating.