Does water become impure if it passes over the vulva after washing the labia majora and before reaching the anus to wash it? And how is the anus to be washed if water becomes impure in this manner?

For a virgin woman (bikr), the prescribed method for istinja' (washing after relieving oneself) is to begin by washing the front private part (qubul). As for a non-virgin woman (thayyib), she has the choice to start with either the front or the back private part (dubur). If the front private part has been washed and the impurity removed, then the water that subsequently passes over it is considered pure because it has met a pure place. Therefore, the water used to wash the back private part is not rendered impure by passing over the front private part after the impurity has been removed. There is no need to pour water from behind the back; this is an excessive and blameworthy burden.