But your original scenerio doesn't involved married, engaged or even seriously committed couples, so I'm still confused as to what constitutes "cheating".
I agree, a married person sexting with another person on a continued basis can be construed as emotionally cheating, even if physical cheating isn't involved. However...just because your BFF's husband states he does NOT consider sexting as cheating is no evidence that he would actually engage in physical or emotional cheating (unless you have definite proof?).
Immature guys sometimes throw comments like that around...makes 'em feel like studs.
(Why do I feel there is more to this story?)