"Probably stopped givin it to her when the kids was born." Pretty provocative statement to anyone, and in cowboy country, I'd say "them's fightin words." But for Ennis--they're lookin at him like they know. So yeah, he's got to prove his manhood. From her reaction, I'd say it's the first time Alma has seen this side of him, but she already knows, from the flipping scene, that he's not all sweetness and light. He'll make her do something she hates. But importantly, she has yet to be disillusioned, so she reacts with shock and fear. Later, that confrontation wouldn't have moved her that much.
Ironic, ain't it, that the cause of the scene, Ennis objecting to their language, is overshadowed when he yells "you want to lose about half your fuckin teeth?"
Can I mention (again) my biggest pet peeve about Brokeback? That the fat, gray bikers are a HUGE anachronism? In the late 60s, they'd be about Ennis' age, maybe up to 10 years older. The biker era was the late 50s, early 60s. They'd be mid 30s, tops. But Ennis wouldn't have had it so easy, given that there were 3 of them, if they'd been young men. They'd have kicked his ass.