Hello and welcome!
Hmm, I'm a bit lost. Does your boyfriend know that you were going on a date? If not, first order of business is letting him know. Or do you have a DADT policy where you don't ask about his dates and he doesn't ask about yours?
What is the background your therapist comes from? How is he with consensual non-monogamy? If you are a self-identified bisexual and desire a same-sex relationship, that just makes you a BISEXUAL, not a lesbian.
Most people experience minor fluctuations of sexual orientation during their life course. Since you have no experience of a relationship with a woman, it's totally understandable that you are curious. Also, once you actually experience what it's like to date another woman, you might feel less interest in men in general, or realize that your feelings for women were more bi-curiosity, and once satisfied, don't re-occur as strongly. Or it might be that you are your happiest, strongest and most fulfilled with BOTH a man AND a woman in your life.
Do you still experience physical desire towards your boyfriend?
Me: bi female in my twenties