This reminds me of a study
I saw: in 40% of the cases, one partner said they had agreed to be exclusive, the other partner said there was no such arrangement.
I think some people assume exclusivity is nothing is stated, some other assume that you need an agreement before you become exclusive (it does seem common to start by dating around and being non-exclusive, and then become exclusive).
Now, this isn't about married couple, but still, it's an example of how expectations can be sneaky and it's good to communicate and create your own rules for the relationship.
And you'd be surprised by how many people think it's "not fair" when they hear their partner, who had a "good job" and made a good living, want to switch careers for one that pays less, and they have to adjust their lifestyle accordingly. As I said, it's usually expected that you will support that choice from your partner, but it doesn't prevent people from resenting it or feeling betrayed.