Jeff's point — that men sometimes have sex strictly out of urgency or pure lust — is not necessarily a cruel one.
Personally, I'm a very sensitive woman and I've always kind of hated the idea that guys are emotionally capable of "love em and leave em". But the very fact that such a phrase exists (plus others like one-night stand, one-shot thing, etc) pretty much proves the longstanding, historic nature of that behavior. If men were truly incapable of such behavior, brothels would never have existed. In my opinion, it's a damn sad thing, and I don't like it. And I think there are men out there who are very selective about who they're with, and require genuine emotional connection along with any lovemaking. But the presence of one kind of man doesn't preclude the existence of other types.
In the context of Brokeback Mountain, Ennis has mistaken the sex with Jack for animalistic rutting. It's part of his denial. ("This is just a one-shot thing" etc.) In truth, their relationship is much bigger and more emotionally substantial than that, for both parties. Ennis only realizes this later.