If the maskana of the gemorrah is that lesbianism is pritzuta b’alma, how can the Rambam say it’s an issur d’oraita based on the pasuk vayikra 18:3? The gemorrah clearly knew that pasuk and didn’t learn it out like that Thank you

ב"ה Shalom This is a very good question, more of a "lamdanut" yeshiva question, nonetheless I will try to make it simple. The source of the Rambam and other Rishonim that makes lesbianism an issur d’oraita seems to be from the Sifra on Vayikra. (ספרא אחרי מות פרשה ח סוף פרק יב) and not from this Gemara. What the Rambam infers from the Gemara in Yevamot is the Halacha that if a married woman did an act of lesbianism with another woman while married she is not forbidden to her husband since the act with another woman does not constitute "znut" which would warrant forbidding her to her husband. In the context of her marital relationship with her husband, we look at the act as "pritzuta b’alma" and therefore she is not forbidden to her husband. But in regard to the very act with another woman , it remains an issur d’oraita as evident from the Sifra. I highly recommend reading the complete Rambam, which I only quoted part of to see what I am saying. It is also brought in the commentaries. Thank you for allowing me to clarify this issue a bit further. All the best.