Is my girlfriend of three years a lesbian?

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I’ve been living with my fiancée for three years but things have taken a turn for the worst. For the last four months we have not been intimate. I have tried talking to her on many occasions but to no avail. However, she sex-chats other men and women right beside me even in our bedroom and I have even caught her watching lesbian porn. Whenever I get another girl in my life she becomes very hostile to them and drives them away. I now have a good lady who knows what I am going through. She has feelings for me and I am also falling in love with her even though I am scared of another relationship. I wonder if it is even possible for a woman to go for four months without sex. Should I take my other relationship to the next level? I have even tried getting professional help but she will not take it. Please advice... {Dan}