It depends on the relationship. If there is consent, then there is little if any difference. Otherwise, cheating is very different from harmless flirtation. Even if you have a sexless union, cheating can inflict pain.

__________________
Lots of people talk and few of them know, soul of a woman was created below

I can't get through to her 'cause it doesn't permit
But I'm gonna give her everything I've got to give.

I hear your sweet voice calling
out my name
As I stare from a six foot cell
And from beyond I heard the words
Deceptively Yours