Most domestic U.S. F cabins have a single lav. Separate M and F lavs would require at least one of:

1. Take space away from galley and/or seats for another.
2. Share lavs with coach pax. (Practical only if there's at least one coach lav forward.)
3. Take a lav away from coach pax. (Ditto.)

None of these is going to fly. Besides, shared facilities maximize throughput because of random fluctuations leading to more M or F usage at a particular moment. If they were dedicated to one group or the other, a queue could build up for one lav while the other was idle.

(I, M, share my home lav with an F. We don't have a problem with it. I suspect most other FTers also share with the opposite sex, or did while growing up.)