yitzchokm
Go back and check your Megillah, the first few times she was forced and the Midrash says her neshama left he body during Biah. However on the other nights when she sought out the King she seduced the King. The Rabbanu Tam (i believe and sure The Rambam both clearly state that she was not forced under that circumstance along with many other rishonim. Therefore Tozfos sites a source along with Rambam that a women is not held punishable if she has relations with a goy. EVEN when she is a niddah. Goyim dont have the mitzvah of niddah and are therfore putar.
I am not saying I agree its a tough pill to swallow in A rav in 5 towns gave a shmuz on this I dont hear so I can say his name but I went and asked a rebbi of mine in Ner Yisrael Rav Singer and he personally showed me the sources.