Cheating?

Brit • Blessings to all women TTC & giving birth. Married to my bestfriend, we have a beautiful 4 year old daughter & 5 months pregnant with our boy 🦋
With a recent post questioning the matter of cheating and what is considered cheating, I got to thinking. You know, I understand completely that everyone's relationship is different, and someone can consider something cheating whilst another person does not. 
What I don't understand, is WHY some women disregard emotional cheating? "Well, they didn't have sex so..." Or "Touching is cheating." 
It just mind boggles me! You know there's more to a relationship than just physical interaction right? Yes. So why say that's the only form of cheating? I don't get it! 
For me, if my man claims to be 100% happy, but is entertaining thoughts of another girl "just for fun" it's still wrong. Is it hurting you? Then he shouldn't be doing it! 
Where did sticking together and being other halves get mixed up? I never even look for other males to talk to unless childhood friends of mine or friends of my SO. Even then, I only talk to his friends when HE is around. 
Share your thoughts ladies, I just don't get it sometimes!