3. Extra vaginal muscles are for more successful intercourse

Because of the shape they have some problems with implantation, fertilization and caring an emryo

That's why their body changed - so every sexual contact will possibly lead to having a child

1. Bicornuate uterus is basically (in this case) just incomplete fusion of 2 separate uteruses (maybe, after some time - probably few thousands years after - it will be a normal massive meaty uterus

2. Extra pair of clitoris' legs means more stimulation

This fanon was borned only because of 1 sentences on avatar wiki, something about 10 kids or something.. so that's why their reproductive system is kinda fucked up 🥲

As you can see there are some differences with human version:
• the most obvious is a shape of uterus - na'vi has partial bicornuate
• they have 2 pair of clitoris' crures aka legs (more stimulation)
• extra vaginal muscles