Why is Cameron annoyed with Christine after she’s been molested?

Cameron was emasculated by his inability to protect his wife.

Seemingly, at the time, he considered himself helpless. Once removed from the situation, hindsight can be cruel. He may have had a means of redress against the officer; he may not. It doesn't matter, because he didn't act.

He is likely not angry directly at Christine; but at himself, at the officer, and at the world which (increasingly) puts a black man at an immediate disadvantage against a white police officer simply on the basis of his race.

Even as a rich, successful man of means; he is victimized by race.

Christine is reminding him of this helplessness.

The issue is that if he were to try and get physical with an obviously racist cop who had a gun things could have gone very very bad. He wanted to do something, he hated what happened, and he felt ashamed that he wasn't able to protect his wife, but there was nothing that he could do. It was a bad situation, but it could have been much worse if he was either shot or if the officer had pulled a gun on him and then continued to molest his wife and take it even further. He was angry at his wife for not understanding that and blaming him for a situation that he had no control over.