When considering the answer to this question, one has to know it’s just an opinion. To stubbornly stamp your answer as the only real/true/valid answer is just pure stubbornness. This is one of those personal conviction type of questions and no one person has the absolute answer for everyone though it seems some certainly try.
Not all believe even the largely accepted idea that any physical activity is cheating – such as swinger, polyamorists, etc. And on the other side there are those who think sharing any emotional connection even with someone half way around the world is wrong.
I strongly believe cheating is whatever my partner (bf/spouse/lover/etc) and I think is cheating. Now that can vary from partner to partner so it’s not a clear one-size-fits-all answer.
I try to open the question by considering the following:
If my partner has explicitly said xyz is cheating – regardless of what I think – it’s cheating.
If for any reason I feel that my partner would be hurt if he knew that I was doing xyz then it’s wrong. That’s where love being about him not me comes in.
If it distracts from my devotion or closeness with him or affects us negatively because I’m sharing myself (emotionally, physically or otherwise) with someone else then it’s damaging to the relationship and should be avoided.
If I feel that I would be hurt if the roles were reversed and my partner did xyz to me then I shouldn’t be doing it either.
If I have to hide xyz or feel guilty about it, it shouldn’t be done because even if my partner doesn’t see it as cheating obviously I do.
In short, open communication about what you and your partner thinks is cheating is key. If you are interested in doing something that may be open for interpretation as cheating behavior then you need to discuss it with whoever you’re with. So s/he can say… well, if you ___ I want to know b/c I think it’s sharing intimacy or taking away from our relationship and maybe we can do it together rather than you finding someone else OR don’t do it because I think it’s cheating and I’m kicking you to the curb if you do OR knock yourself out it doesn’t bother me… Hell, I might even do it when you aren’t around OR however it is s/he feels about it.
As for what those outside your relationship think… screw ’em.