Back OT, it seems we spend a lot of time discussing Ennis's se4xuality, but don't say too much about Jack's. Surely Jack was more sexually active homosexually, but he, too, denies he is queer to Ennis. What is Jack's fear?
I don't think Jack was afraid as such. I think when he denies that he's queer to Ennis, he is holding back, knowing that if he tells Ennis the truth then there's every chance that Ennis will stop their physical relationship from continuing. I think he's already pretty sure about what he wants and needs, but he knows that Ennis isn't. Ennis states catagorically that he's not queer so intead of risking everything, Jack agrees with him, letting Ennis think that this really is a one shot thing, between two very good friends, with nothing more to it than that.
Once back in the real world, Jack tries to pick up other men (Jimbo the rodeo clown) but fails and so, when Lureen comes on to him, he allows it to happen, wanting some sort of physical connection with another person, even if it's not what he really needs (there is, of course, the added attraction of LD Newsome's money - but that's a whole 'nother discussion!)
After he reunites with Ennis, I'm guessing that he probably had no real physical relationship with Lureen - this, I think, is shown when he only gives her a perfunctory kiss when he's leaving to go off on a trip with Ennis. If there was something more in their relationship, it surely would have shown when he was leaving for a week?
And then, of course, we know that he starts to become more sexually active with men after Ennis' divorce - by which time I think he has realised that Ennis isn't going to change, so there's no reason to hide his sexuality any more - no more risk of frightening Ennis off, because Ennis is already frightened enough.
Of course, he even ends up admitting it to Ennis....which, I think, is the final confirmation that he's always been "that way".
Just my 2p worth, mind....