Ooh. I'll jump into the pool hip-deep.
Here's what always confounds me just a little. First of all, I agree with you, Ray, that Lureen is certainly onto something when she says "Huuusbands... Don't never seem to wanna dance with their wives. Why do ya think that is, Jack?" And yes, I think Jack knows she's onto something and so whisks LaShawn away from the table, not to playfully spite his wife, but to distract her from what she may be getting more and more onto.
Back to what confounds me (yes, I remember!) When Ennis first calls her and says "This is Ennis Del Mar," she doesn't quite have the true recognition of what he meant to Jack yet. She says, "You're that hunting buddy, or fishin' buddy, I know that. I'da called ya to tell you what happened, but Jack kept his friends' addresses in his head." So there, she's lumping him in with any of Jack's other friends. THEN, he says, "Me an' Jack - we herded sheep up on Brokeback Mountain in the summer of '63." RIGHT THERE. That's when she knows. Because Jack said it was his "favorite place." She thought it was maybe make-believe, or a place to go drink. After all, Jack drank a lot... Right there is where she puts 2 and 2 together.
So does it follow that she always suspected it was a man and not a woman he loved more than he could ever possibly love her? In a way, yes, I think so. Otherwise that epiphany wouldn't have come so swiftly. It only took a moment for it all to click. I guess I picture her always feeling Jack's indifference and wondering is it me, or is there someone else? When you start to think of the possibility of someone else, you start to watch your husband's interactions with women very closely. Is it her? Is it her? Is it someone like her? I think of Lureen watching this and realizing that Jack doesn't seem to show the least bit of interest in other women. But he talks about that Brokeback Mountain all the time. How he spent the best summer of his life there once. How he wishes he could get back up there someday. All the time. He goes on all those fishing trips, but he never goes to Brokeback when he does. Or so he says. Hmmmmmmm... And I can see him never mentioning Ennis by name to Lureen, because let's face it, you don't do that when you're in love with that person. You're afraid your face or voice will betray you when you say his or her name. So you don't. Hence her not putting the pieces together yet just from Ennis saying, "This is Ennis Del Mar." But he mentions that mountain, and BLAMMO.
I love Lureen. I think she adored Jack and it broke her heart that she could never have him the way she wanted, similarly to how he could never have Ennis the way he wanted. We know she loves him by that comisserating look when L.D. says Bobby is the spittin' image of his grandpa and by the little smile when he finally stands up to the son of a bitch. I think Jack very simply married her for her money because he was tired of being dirt poor and because he didn't want to keep being vulnerable to the Aguirres and Jimbos of the world - he thought marrying her could save him from both of those fates. But she married him because she loved him the moment she laid eyes on him. Hell. Who wouldn't?