I think Ennis is standing up for himself the only way he has learned how. He is putting up a shield.
Yes!

But when twisting her arm after the 'Jack Nasty' comment
he does this he is "protecting" not himself, but Jack. No way does he want anyone talking badly about the man he loves.
Good observation! Of course, Ennis is hurt by the Jack Nasty comment. But this scene is about a lot more than Alma calling Jack names. It's one of the most crucial, devastating scenes in the story. Alma finally plays her trump card and reveals to Ennis that she knows his big secret. Worse, she didn't just figure it out the other day, but she's known for years! Ennis totally freaks out, he is at once humiliated and horrified -- Has Alma told anyone else what she knows? If people find out, Ennis (not Jack who's far away) is in mortal danger. That's why Ennis warns Alma "You don't know nothing about it" and threatens to make her and Monroe eat the floor, etc. etc.
But it's fair to say that Ennis's outburst is at least, by extension, an attempt to protect Jack.
Just another point then I'll go to my corner and be quiet -- it's in the next fishing trip following this scene that Ennis is all paranoid and asks Jack if he ever suspects that people know about him. And shortly after that he takes up with Cassie to be his cover. From one lie to another...poor old Ennis. :'( :'( :'(