I think the difference here is that in general, people date with the intention and expectation of finding a spouse, with whom to grow old together. Obviously, there are exceptions to the rule. Obviously, it doesn't always work out that way. But there's some expectation and goal of that.
In secondary or outside poly relationships, it's much more likely that there's an expectation of impermanence, or at least little hope for permanence. Again, obviously there are exceptions.
But I'd be curious to know how many here go into a secondary relationship thinking, hoping, expecting, or planning that it might last a lifetime, that you might grow old with this person.
I would further point out that in the typical primary or mono relationship, actions are taken that require commitment, that show that expectation of building a life together, that entwine lives: buying a house together, getting bank accounts together, having children together.
This is much less likely with a secondary relationship.