I imagine that when they meet they must have a decent amount of sex, though, right? They ride, they fish (maybe?), they cook their meals, they drink and talk and sleep. so when Jack says a *couple* of high-altitude f&*ks, he really means that compared to the rest of the long days in their lives, this is very little. But they're packing as much as they can into this time--he just wants more, and believes that Ennis does too, although he can't admit it.