Hi !
As virility increases, the time to erection decreases, which is why the number on the bar goes down.
For now, it doesn't change much, except in certain scenes where the character is either erect or at rest, which can lead to comments from other characters. I'm planning to introduce a malus in the future that will affect the number of sexual encounters the main character can have in a day based on his virility. If his virility is too low, it could lead to the girl giving up or impotence.
Stamina affects how long you can last before ejaculating during sex. If you have a lot of stamina, you can usually play more scenes, or you can repeat scenes several times. However, if you have too much stamina, you might occasionally experience some sort of pleasure denial.