There is no absolute definition of cheating. It's a little different for each relationship. However, the general rule is that if you feel you need to hide it, then you are likely cheating.

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Put me on wheels and I'll turn tricks.

Clever? Nah, I ran out of that years ago. But if you find this, let me know, k?
"The road goes ever on..." ~ Tolkien

In memory of my friend skip...
Go then, there are other worlds than these