Ahhh - thank you for explaining further.
For me, I think the chances of ending up in situation number 2 are drastically increased by assuming that any relationship will remain the same romantic relationship for a lifetime.
Anybody who feels that they are tied to somebody else for the rest of their life is, I think, much more likely to put the sort of pressure you talk about on their partner.
I think it's that sort of belief in an unchanging relationship between unchanging people that causes those sorts of problems rather than the shape of the relationship.
For sure, there are lots of stories about mono folks putting pressure on each other to conform to each other's wants. There are also lots of stories on this forum of poly folks doing exactly the same thing. As well as lots of stories about both mono and poly folk treating each other very well and dealing beautifully with mismatched life goals.
I don't think it's the shape of the relationship that is the problem. Being open to the fact that no relationship is likely to remain the same for a lifetime is, I think, key to them ending in situation 1 if they do indeed end.