So *loved* this scene.
If we try to place ourselves in the minds of the characters, I think neither of them could expect anything. I think they both hoped though.
They only traded two cards, so neither knew much about the others life. Jack could probably infer that as Ennis was in Riverton he married Alma. I guess it's puzzling why Jack left it 4 years to get in touch. Ennis had more or less given up on Jack.
Whatever their expectations of the reunion was, I think, as soon as they saw each other, they knew, or at least reaffirmed that they were meant to be. Just as the right key opens the lock. (The lock being Ennis). This scene shows Ennis as his most happiest, his most passionate. If I was Alma witnessing this, I'd be equally surprised and hurt that here was my husband acting with a passion that I'd never myself witnessed as much as the fact that it was a man he was sharing this passion with.
The tragedy of this scene, is although they both reach the same conclusion - they have to be together, their minds process that in similar ways. For Ennis, its how they can be together, causing themselves, and their responsibilities, the minimum of disruption, and retaining appearances for everyone's sake. For Jack, its just that they must be together.