My apologies for the long winded response. I just find a lot to consider, and a lot to relate experience with.
First off, I get the feeling it's for the sake of acknowledgement on Jack's part; to show Ennis he's not giving up on him (yet); to not corner him. Jack is close to pushing Ennis away, probably has been for years. Meanwhile, it is eating Jack alive. "Sometimes I miss you so much.." is proof of the appetite of Jack's love that he can no longer satisfy with emptiness; the next day he rumbles and growls at Ennis from the inside out. It's as though Jack instinctively let's Ennis understand that he's been keeping him on a leash and yet, starving him. If only Jack knew how to snap the leash and go find proper nourishment, love, he is saying he would do so; instead, he was loyal to Ennis. He was prisoner to love.
This said, I think the night prior is a thinning prelude to the finale of their chemistry, an extended, lit fuse finally bringing ignition and POW -- but of course they explode together. This echoes many a time including first departure, divorce scene, and this final confrontation when they recoil in an embrace at the ground (nevermind how I think afterlife will reunite them after the explosion of Jack's death).
"All this time and you ain't found anyone else to marry" / All this time and you still love me the most?
"All this time and you ain't found no one else to marry" / You're still content to be stuck in limbo?
There are many double meanings, mainly aimed at Ennis' psyche and the likelihood that his transparent walls will finally crumble in the favor of his and Jack's love affair. Personally, I don't lean on the notion that Jack was insecure about Ennis' feelings towards another woman or man. Rather, he was painting a contrast between their lives off the mountain, and the love between each other, while suffocating on the fumes of red paint. - best to use *images* on Ennis.
Ennis: .. I been puttin' the blocks on this pretty little gal up in Riverton / I still haven't forgotten to act straight.
I find it beautifully bittersweet that Jack couldn't outright "hurt" or jeopardize Ennis by saying he'd been considering Randall, and had to throw in "fooling with" his wife. It hurts to know how easy it is to talk of their empty 'straight' lives than it is their own love; and these tiresome facades had been getting to Jack and the longer he was left in life to feel robbed of his true love, the shorter that fuse got. He sleeps on it, and in the morning with the mere mention of November, he is set off.
All this time and you ain't found no one else to marry..
20 years ... and you still haven't turned 10.
:\'(
Yeah, I think Jack needed an "I love you". Ennis could have stomped the fuse out, had he been able to offer something sincere. I think that's what it boils down to ...
For now, that's my 2 cents.
Forget the apologies for long-windedness, I can relate to the passion , believe me you

and this speaks to what I've considered about this question.
I think Jack was searching for something, perhaps that he is still the one and only in Ennis's life. He would love to know what's holding Ennis back, a woman? Nah, women aint considered serious competition in this love affair. Been there, done that.
He does know the solution to his problem lies within Ennis himself a tough piece of work. Had Ennis said he was putting the blocks to some fellow co-working cowboy, Jack would've been shatterd. Of course that was not the case, but " after all this time" , Jack is seeking confirmation for himself.
Yes, quitting Ennis has already crossed his mind, but he still has hope.There is definitly a connection between this scene and what transpires the next morning during which as you put it " a fuse has been lit" long time ago in Jack.The noose on the short lease is getting tighter and tighter around his neck. Yes, Ennis could've stomped out that fuse or cut the noose some slack just by uttering " I know the feelin" but that, imo opinion would not be good enough. He needs to hear something more substantive. He is already trying to prepare to quit,...with Randall in Childress waiting in the wings. ( a direct threat to Ennis, though that's open to speculation...) but anyway, Jack spares himself and Ennis THAT drama and opts out for LaShawn, Randall's wife. A lie, but it achieves the desired effect, big deal, Jack is screwing around with some ranch foreman's wife.
All them things Ennis don't know can't hurt him, but it leaves Jack very vulnerable to outside influence. Randall living in close proximity, and the Mexican border close by. . I have concluded that :.. "all this time, you aint found nobody else to marry? = after all these yrs, I need to know that I'm still the one you love. Jack is desperate for answers from Ennis involving hope, confirmation , and inspiration . With direct, open communication lacking, Jack says it all with " Truth is, sometimes I miss you so much, I can hardly stand it" . It's Ennis he really wants in his life, It's Ennis that he really needs despite his daliances. Finally, it is this statement, along with " I wish I knew how to quit you" that goes to the essence of the problem between Jack and Ennis, it speaks to the tortured dilemma in which Jack finds himself. MHO.