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She isn't being stupid, she is thinking rationally about the future. She is a slave, a marriage with trigger would protect her as she wouldn't be sold off to someone else. And she can trust trigger since he just want a fake partner as he is gay and need this so he wouldn't end up being married to a woman who want a real husband.

As for the Duke she don't really know what he want or is thinking, he didn't outright said he loved her, he kissed her. For all she know he could just have been fooling around, slaves can be playthings. Also as the Duke his partner is important, is marriage has political weight so he shouldn't be able to have a slave for his wife, at best a mistress. So it's only natural for her to rule him out of her future as she don't know she is the main character of a romance story.