1. Physical (which you said you understand)
2. Mental (sharing thoughts/ pieces of yourself with somebody other than your boyfriend/girlfriend even if you technically don't have sex with them. i.e being closer mentally and emotionally with someone else)
Consciously choosing to do things such as frequent strip clubs etc i.e paying someone to strip or whatever. I guess it approaches the definition of mental cheating, except that there is not one individual you are close to or anyone you would cheat on your gf/bf with, but just ... I guess not giving your relationship the respect it deserves? i.e if you are not aroused by your SO, why should you even be in the relationship?
You were the one who brought up conscious and unconscious straying, perhaps in response to your original interpretation of what I was saying. Also, I said I would not break up with someone over my definition of "straying" but I would have a problem with it, and if it was a continuous issue we would probably break up.