Well, like many here probably, I have seen the film more times than necessary, and in fact do have it memorized now.
I don't believe Lureen knew Jack was gay, when they first met. Hell, as horny as she was, he could have probably told her there in the back seat
of that Thunderbird, "well, you are in a hurry, aren't you.....you know, I'm queer",
and Lureen would have still responding that she had to have the car home by midnight, while mounting him!
I think by the point in tiime, where Jack and Ennis are last on Brokeback together, and Ennis asks Jack, "What about you and Lureen?"
I can tell by the time that Jack takes to take another hit on the bottle and the expression on his face from the side, as
he says "Lureen's good at makin hard deals in the machinery business, but so far as our marriage goes, we could do it
over the phone."
That one line there by Jack tells me that his marriage has gone south. What his sex life with Lureen was like after
fifteen years of marriage is anyones guess. My guess is with periodic "fishing trips" with Ennis, and whatever Jack called them
trips to Mexico, I doubt there was much fire in the sexual department of their marriage.
If Lureen had an equally low sex drive, she probably didn't suspect anything. If she expected sex in the manner that she
did back in the mid 60's, then yeah, I would think if Jack didn't put out for her, she would suspect something.
As someone else posted here, and this is just excellent acting and direction on Ang Lee's part. When Lureen gets the
phone call from Ennis, and he tells her, "we herded sheep up on Brokeback one summer, back in 63", you can see the expression on
her face change, and she puts it all together in that one scene that this is the guy who has been getting Jack's
lovin all these years.
That's the way I interpreted it.