I guess most of you will disagree with me but I actually see the beginning of this scene as being a positive step for Ennis.
I think that he is beginning to attempt to start trying (phew!) to define his relationship with Jack. I have always said that I think Ennis started ‘this thing’ as a homophobic man trained to be so by his father. When his relationship with Jack turned sexual, I don’t think that he spent time thinking about what it meant. I think that he was scared to think about it because of the inevitable conclusion, so any thoughts about Jack were deliberately pushed out. He loved Jack and missed him (which could be, if you want it to be, rationalised by saying he was his friend and they were alone on the mountain with needs).
Around the campfire, after the reunion scene, he refers to what they have as “this thing [that] grabs hold” as if it is an object, something that they can’t control, with a mind of its own. By asking Jack about others looking at him “like he knows”, “like they know too” he is referring to that they have as being a part of him, i.e. while looking at him what do they know about him?
Secondly, Ennis immediately rejects the move to Texas by mentioning his girls. Ennis is a man of few words and I think that anything he says is to be taken at face value. He doesn’t, at any point during this conversation, say about it being unsafe. He talks about the long move from his girls. He is reminding Jack that it’s not just about them, they can’t just live together as if they were on the mountain as young men. They have responsibilities now. Jack hasn’t got it all figured out, like he thinks.
Regarding the plates flowing down the river, Ennis was holding onto the plates and during the conversation, lets go of them. He was, for the first time since the reunion scene, entering into a serious discussion with Jack. He was attempting to let go a little at which point, Jack grabbed on. He asked one question and the conversation ‘ran away’ from him. The plates, to me, are more about Ennis’ fears of letting go, as if he does, who knows where he'll end up?