Before I reply boo_boo, I think I should get the quote right now that I've got my copy of the story here...
They never talked about the sex, let it happen, at first only in the tent at night, then in the full daylight with the hot sun striking down, and at evening in the fire glow, quick, rough, laughing and snorting, no lack of noises, but saying not a goddamn word except once Ennis said "I'm not no queer", and Jack jumped in with "Me neither. A one-shot thing. Nobody's business but ours"
I think the key phrase (and the reason I felt I should reproduce the quote properly) is "Jack jumped in". I think that Jack was trying to make Ennis relax about what was happening, knowing that Ennis was still very unsure and edgy about their relationship, knowing that if he (Jack) said he was queer then Ennis would most likely back off completely. Even though the words come out of different mouths in the movie, this feeling is still the same - watch Jake's face when he says the "Me neither" line because he plays it to perfection, responding quickly then flashing a quick glance at Ennis to see if he's picked up on the lie.