I watch Lesbian porn and I'm in a heterosexual relationship. I find women attractive but I'm not a closeted lesbian. She may just be exploring that side of her. Unless she completely detaches from you or you suspect me, I'd just embrace it.
My boyfriend kind of enjoys that I can judge women based on their attractiveness with him.
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Hope you get it sorted out. Don't be afraid to tell her your feelings. Have a talk with her see what is up.