She is correct. She is free to choose her behavior.
It could true -- she may need a dom that is not you to be able to keep the power exchange in a healthy psychological balance for her.
But the thing is... she is NOT a footloose single here! What she does affects her husband -- YOU!
And if she chooses to behave in a way that is lacking in compassion, respect, and loving kindness to her husband? She expects the husband to live a double standard? She expects the husband to just deal with whatever her actions are? What could the husband do?
You are free to choose YOUR behavior.
I do not mean this in a bad or insulting way, so I apologize in advance if it seems rude. But is she fit in her mind? Is she acting out of character with all this recently or has this been present all along? (Sex addict? Psychological damage or disorder? Something else?) If you suspect she is not right in herself, could choose to ask her/take her to get checked out.
If she is healthy in mind and body -- Could choose to seek a divorce lawyer. Let her go. She's fit and sound and she chooses to mistreat you!
I am terribly sorry you are going through this.