The more I watch the thing, the more it seems to me that long before Jack's death she must have "known" inwardly but not said anything/acknowledged what she knew, because it was unspecific.
What in the movie, Welshwitch, gave you the impression Lureen knew about Jack and Ennis? I can only remember 4 times we see Lureen after she and Jack get married.
One is when the two men were in her office calling Jack a pissant and Lureen doesn't even defend Jack. She never even said, hey fellas. That is my husband your are talking about. She just goes back to work.
Another is at Thanksgiving when she sits there and giggles to herself because Jack just told her old man off.
Another is when Jack is in her office looking for his blue parker and they "kiss." She didn't seem concerned Jack and Ennis were getting together as much as she seemed "concerned" that it was always Jack driving 28 hours round trip.
And the only other time I remember her is at the dance when Jack asked another man's wife to dance instead of her.
To me, none of those scenes showed she knew or even suspected. Was there something else I might have missed?
Obviously I can't speak for welshwitch, but personally I think that all those scenes you mention feature little hints that together, over the years, would have made Lureen at least suspect, if not know for sure.
The scene in the office, unless I've totally misinterpreted it, the two guys were implying that Jack was gay, with the comments about how he used to try, to ride the bulls. It always makes me think of the scene with Jimbo, where he says something similar, and also the line from the Motel scene in the ss, about Jack "riding more than just bulls". Like I said, it may just be that something's got "lost in translation", but I thought "riding the bulls" was an euphemism for having sex, or at least trying to, with men
The Thanksgiving scene, when LD says about Bobby growing up to be a man, he obviously doesn't think Jack is a "real" man,and is implying he's gay, and however much Lureen enjoyed seeing Jack stand up to LD, she must have wondered if there was some truth in what LD was saying.
The blue parka scene, I think Lureen is fishing (If you'll excuse the pun
:-[ ) for information about Ennis, asking why he can't come to Texas and fish,she wants to know what's so special about this guy that Jack's prepared to drive all that way just to sit around fishing, when they could presumably do that anywhere there's a river
When she says about the new tractor coming in that week, it reminds me of Alma warning Ennis he could get fired for taking off with Jack after the Motel scene, trying to keep him home by reminding him of his responsibilities.Of course there's a difference, in that Alma knew what the fishing trip was really about, so I suppose it could just be that Lureen was annoyed form a business point of view, but I get the feeling there was more to it than that, I don't know what exactly, female intuition, maybe?

I think if you add all that up, and add the fact that the physical side of their relationship was practically non-existent by the time they meet Randall and LaShawn (I'm guessing that was the case from the reunion onwards), and the fact that Lureen had to make all the moves when she first met Jack, I can't see how she could have not suspected. I don't think she realised untuil the phone call, just how strong the bond between Ennis and Jack was,but I definitely think she already knew that Jack was gay, and that his "fishing buddy" was more than just that.
CrimsonSky, you brought up some points that made me think. So, I went to the movie and watch the 4 scenes again, 4 or 5 times. Except for one of them, I still can not see anything in the movie that indicated Lureen suspected. Lets start with the two men in her office.
One says, Ain't that the guy who use to ride the bulls. The other says, he use to try. Ok, so he wasn't very good at bull riding. But he did win a buckle and we did see him win the day he met Lureen. Even Jack sort of admitted he wasn't very good when he told Ennis the Army wouldn't take him because he was too busted up. But were the two men suggesting Jack was queer?
I don't suppose people outside of the US get to see much of rodeos but it takes a fearless man to ride the bulls and broncos. Even if you are gay you got to be a man to get onto the back of 1500 to 2000 pounds of bucking beef; beef that can stomp on your head in a second. More than a few men die every year riding the bulls. These two men know that. And the fact is many men try and fail at riding. So were they trying to suggest Jack was gay? I just can't see it. Even in my prime you couldn't get me to sit on the back of a bull in a tiedown pen and I was a pretty adventurous person. I admire rodeo riders. And some of them are pretty good looking hunks. Oops,

What disturbs me most about this scene is Lureen. It was obvious the two men didn't come from or around Childress or they would have known Lureen and Jack were married. Perhaps that is why they made their remarks in front of Lureen. But Lureen never tried to stick up for her husband. She was already at the stage where she didn't care about him. All she had to say was three words; that is my husband, and she would have humiliated them. Instead, she just goes back to her work.
Now, as for the two men using "riding the bulls" as a euphemism for having sex with men. That is a very good point. But, Jack must have been a little whore approaching every man he met in order to have gotten that reputation. And what about Lureen? In 1960's and 70's Texas I don't think homosexuality was talked about very much and definately not in front of a young woman. Would she have understood the euphemism if she heard it?
The Thanksgiving scene; L.D. sure was referring to Jack when he made his "grow up to be a man" remark. But Jack proved he was a man by standing up to the old Stud Duck and telling him he would kick his ass into next week if he didn't shut up and sit down. What did the old Stud Duck do, he shut up, tucked his tail between his legs and sat down like a little boy who had just had his ears boxed by his father. I think we saw who the man was there.
And for you ladies who think watching football makes a man out of you. I am here to tell you that watching big calves, big thighs and big arms on men wearing skin tight pants is a little faggots dream......an old faggots dream also.
The blue parka; well, I don't think Lureen cared that Jack and Ennis were going to be together as much as she was concerned it was always Jack paying the cost of going up there. I think she was trying to squeeze another zero out of the numbers. I mean look, it was ok if Ennis came down there and they went fishing.
Back in the 70's there was a shortage of gasoline and it was expensive. Could it be that is all Lureen was concerned about?
But you know, there is something else that makes no sense about Lureen complaining about it always being Jack to go to Wyoming. Lureen knew Jack would visit his family while up there. How could he visit his family with Ennis coming down to Texas? No, had to be that extra zero she was trying to find.
My word! The things you can think of that are so

. Why did Jack never bring his family up to see his parents? I would think that would be one of the first things he would do. Here pop, I am a man and here is my family. And I make a good living and they live in luxury compared to you, you old son of a ...... Maybe he loved his father too much to do that to him. Hell, Jack could even have brought his family and a ton of groceries up to visit his parents while he and Ennis went fishing.
Ok, ok back on topic.
There is only one scene left
CrimsonSky, and I just might give you a point here.
It seems to be a big deal that men don't want to dance with their wives, or Jack doesn't want to dance with Lureen. Lureen could have noticed that Jack and Randall were engaged in an awful lot of eye contact and smiles. Suppose Lureen's question, "Why do you think men don't want to dance with their wives." the word dance was a euphemism for have sex with. Could Lureen have suspected something watching these two men eyeballing each other all night and smiling? Possibly.
Or, was Lureen just trying to save face in front of these other people and trying to get Jack to dance with her to make the others think their marriage was normal? Either way, Jack sure put her down by asking Chatty Kathy to dance instead of her. The ultimate putdown in front of strangers.
But because of the possible euphemism and that euphemism coming from Lureen herself, I am going to give you a point there
CrimsonSky. But, if she did suspect at this point, and it is a big if, this is the first point at which she suspected.
OMG, and if she did suspect, and it was with Randall who lived in Texas, could this reopen the "Did Lureen have Jack killed" thread? Imagine, Jack taking of a week or two with Randall AND Ennis, plus weekends with Randall. What was left for Lureen?
There is only one thing more that you mentioned that I would like to comment on. The back seat of the car.
Jack saw an opportunity to marry into a wealthy family and to be comfortable himself. Lureen saw a man her father wouldn't approve of and to get back at him did what she could to get pregnant knowing her father would make Jack marry her which would have been a double blow to him.
Of course, she could have already heard the rumors about Jack and this would have been yet another thorn into her father's side. But, I am convinced she WANTED to get pregnant in the back seat of that car. For, whatever reasons.