She is whatever the player wants her to be.
I don’t want to specifically define her sexual orientation. It limits writing scenes with her. Additionally, keeping it loose allows the player to project what they want on her.
Ok, you sort of dodged the question... but I'm fine with that. I guess my headcanon is that Mila is bisexual: she fancies men, but wouldn't mind the right (or wrong) woman.
And by the by; the scene that inspired me to ask the question was when the Match Maker (from the "Tragic Bride" contract) tries to hook Mila up with one of his clients, Mila points out that almost all of them are women.