How's this.
Think of your significant other, ok.
Imagine he/she has a friend, an acquaintance, that you do not know.
Now what would he/she do, what behavior would occur that you would consider "cheating on you?"
Heartfelt letters?
Cybersex?
Some heavy petting in the back of an Acura?
What is the standard of intimacy that you would set for them, this imaginary couple?
Whatever you would find intolerable and "too much," well, there it is,
that is what you consider cheating.