(Oh this is a good one. I had seen BBM just once before the long wait for video, during which I took in all the discussion on IMDB.)
On just the one viewing, I was positive that Jack was in no way interested in Randall. He did seem to pick up Randall's vibe at the dinner table. But when Randall asked on the bench, Jack seemed as if he'd just been offered Brand X beans — and his expression was a mix of shock at Randall's ability to be so brazen, and a faraway 'missing Ennis' look.
I stuck by this impression a long time. (iirc, Littlewing and I were the main holdouts swearing there was nothing, ever, between Jack and Randall!)
I was eventually swayed by other folks' observations and by subsequent BBM viewings. Obviously, Jack has no chance to (verbally) respond to Randall in the scene we see. But latter parts of the film (OMT's mention of the 'ranch neighbor' etc.) indicate Jack accepted the "fishing trips" offer, although it was a pale second-best to what he wanted with Ennis.
The "wanna dance?" in the dining room? The excellent way it was shot deliberately makes it vague and intriguing. But after repeated Zapruder-like reviews of the scene, I do believe Jack is looking at LaShawn.
The trick that makes it look otherwise: when the camera cuts to her and Randall, Randall is looking straight at Jack but LaShawn's eyes shift from Lureen to Jack — making Randall look like the 'recipient' of the question for that split second. But in the previous cut, Jack is looking at LaShawn. (imo)