I understand and agree with your definition of cheating, but what constitutes a partner? One with whom terms have been discussed? Do I have to be the one to discuss them if I really don’t care who these girls are sleeping with other than me, but I’ve inferred that they care in regard to me?
A partner is anyone with whom you have a relationship with mutually understood and agreed upon terms. That relationship can be based on love, business, crime, or whatever. In the context of love, the terms are usually based on fidelity, which centers around acts of intimacy.
The key here is “mutually understood and agreed upon.” There is no partnership otherwise. You can have a relationship, but until you both establish and agree upon the terms, it’s not a partnership.
Of course, the terms can be anything you want. People have all kinds of arrangements, and when it comes to romantic partnerships, those arrangements usually focus on some kind of physical or emotional exclusivity.
Unfortunately, people don’t communicate very well in this aspect of their lives. They assume shit. They infer shit. They apply rules from prior relationships without really discussing it, and what passes for tacit understanding for one person often flies right over the head of somebody else.
That muddling of expectations is where you run into problems, so yes, if you want to avoid the potential mess, you should take it upon yourself to be the one to discuss the terms in whatever manner you deem most appropriate. If you don’t give a fuck either way, then fine. That’s the risk you take.
Again, there is no right or wrong here. Shit doesn’t have to be all cut and dry. Hell, it never really is. I’ve just always found that it’s better to be open, honest, and up front with expectations.