I calmly told her that while she was in a relationship with OM, I didn't want her staying at the house. I couldn't legally kick her out, so made sure to word it in a way that conveyed my feelings towards her staying there. She did not threaten to take the kids or get a lawyer, but said she didn't think she could ever forgive me for what I had done. She still holds some anger towards it, but I've recently seen signs of her coming around.
My hope is that in the long run, she'll come to understand why I did it and respect me more for standing up for myself.