I understand your point. Use disappointment as method to discover that you should let go the relation and move on. Its a way, maybe a valid one, but you can do it with less suffering.

You can have a relation without disappointment and expectation and in a natural way discover you should part away, it just will be with lot less pain.
We shouldn't have the excuse of suffering as a reason to end the sexual relation. You can always end sexual relation and still like the person as friend, as brother.
If the relation is deep and true, them it may continues as friends if they at some point discover they want something else, or someone else.
If the relation is not deep or never true. Then just realize it, feel it, and move one without hard feelings.

Suffering is need for evolution, suffering it says "you need to learn something" but when you learn it there is no more need for suffering.

Little signs will always tell you if you should continue the relation, they came naturally, no need to expect behaviors to determine if the other one loves you, or that you understand each other in the way of living.