Before you come here and tell me that every case is unique and needs to have an honest conversation, I totally agree with that and people should be more explicit about their needs and goals in life in the first stages of dating (marriage, kids, religion practices, etc.).  

But it seems to be the case that there're ambiguous rules that seem "obvious" and often times not brought up because of that. If you put the obviousness level in a spectrum, in seems pretty obvious that people usually don't mention that they don't want to date a person who would murder them, or a person who would murder their child, because it's usually implied (and often times not even thought of) that that's the case. And if you go to the other side of the spectrum, things like common interests, age and how many people do they live with is often brought up, because they're not obvious and people can't risk to assume.  

Now, about the exclusivity. It seems to be the case (and I'd assume most of you would agree with me) that in the past (50-100 years ago) the exclusivity from the point you start dating someone was the default state of every relationship (unless specificied otherwise explicitly), and was on the obvious part of the spectrum, where you wouldn't mention it explicitly (just like how you wouldn't mention that you don't want to date a murderer).  

In that case, the onus is usually on the person who wants a non-exclusivity to explicitly state that they want it. And failing to do so would mean they breached the trust (since they broke the implicit rule, just like how a cheater would break an implicit rule, even if it wasn't brought up explicitly).  

But nowadays, it seems like non-exclusivity has starting to become a default state for more and more people (correct me if I'm wrong), and to be exclusive from the start of the dating (which was the default in the past btw), now the onus on the person who wants to be exclusive to explicitly state it.  

If that's become the case, why do you think that has changed? I see a lot of posts where people complain how they didn't explicitly state that they want to be exclusive, and their partner assumed the non-exclusivity and have slept with other people while they were dating.  

And btw, I'm not condemning any type of relationship, and if people have an honest and open communication with their partner, they're free to have any type of relationship. I'm specifically talking about the implied rules (like "don't cheat on me", "don't murder anyone", "don't have children with other people", they're all default)  

TLDR: in the past, the exclusivity from the start of dating was the default and you didn'thave to mention it explicitly, but now it seems to be the case that for more and more people the non-exclusivity has become a default, and people who still assume otherwise have been hurt, and almost feel like they've been cheated, but their feelings are being invalidated by people who think that the non-exclusivity is the default.