I agree with most of what you said, Amanda. And Jeff, I can't quite figure out how Ennis gets the bloody nose, either. (In the story, isn't the whole fight reduced to a much later, mostly unexplained reference to the punch?) So here are my other random thoughts:
My take on the punch is that it's a combination of impulse (Ennis just got physically hurt and his reflex is to hurt back) and his anger at Jack for not being more upset about leaving the mountain. IMO, it's not, as some have said, about his confusion over his own emotions, or the need to get them under control, or the desire to start separating himself from Jack and going back to his straight life -- but bewilderment that Jack appears not to be be distraught about leaving, as he is. So I guess actually both reasons could be described as "Ennis just got hurt and his reflex is to hurt back."
For the next four years, I think he misses Jack like crazy and worries -- assumes -- that he'll never see Jack again. He probably goes over and over the punch in his mind, not only because he fears it makes it even more unlikely he'll see Jack, which he's probably not very hopeful about anyway, but because he's sorry they parted on that note. When he brings it up in the hotel, it's in the context of also asking about Jack -- using the exact same words Jack had used when they parted: "So the army didn't get you?" Obviously he's never stopped thinking about that moment.
My reading of Jack is that it's long been all but forgotten. "Well, the next summer I drove on back up to Brokeback" -- he's changing the subject and telling Ennis how much the punch didn't bother him, that he just wanted to see Ennis again.
You're right that if Ennis punched Alma and then Alma's face was bruised, we'd think of it differently. I don't think this is sexism or a double standard on viewers' parts, though, it's a matter of the balance of physical power. Obviously Ennis could beat up Alma if he wanted to. Could he beat up Jack? Maybe, but it would be a lot closer fight. Jack is a tough guy, he's used to getting thrown off bulls and getting busted up. He gets thrown by his horse, he crashes backward over the buckets when he's goofing around. And he and Ennis were wrestling in the first place, the wrestling turned more intense and serious on both sides, and Jack did inadvertently hurt Ennis. So we as viewers know we needn't feel too sorry for Jack over one punch. That doesn't make it OK, and if it kept happening, that would be different. But in all other times (except for the lakeside argument), Ennis is gentle and protective with Jack.
Also, it's not like he's going all out to really injure Jack. I think one reason for the 4th of July scene is to show the contrast between Ennis' fight with Jack and the way he fights with men he's not in love with.