I think it depends on the situation. My friend's boyfriend once kissed me in front of her as a joke (massive SMOOCH on the lips) and I found it really awkward but she just laughed but if it was with anyone else or behind her back, she would have decked him.
I don't think you can define cheating so simply because each situation changes and it depends on what the couples have discussed. Like I think I could do a multiple relationship provided there was clear communication and lines drawn but if someone I was dating came to me and said 'I just slept with soandso but it's not cheating because you said you wouldn't care' without asking first, then yeah, trouble.