Does the mixing of semen with ritually impure substances that are excused—such as a drop of urine or prostatic fluid—render it impure, even if the semen itself does not change thereby? And can this be analogized to the opinion of those who do not consider a small amount of water impure unless it changes?

It is not permissible to abandon istinja' (cleansing after defecation or urination) under the pretext of waswasah (whispers/obsessive thoughts). Rather, istinja' must be performed with water or by istijmar (cleansing with stones or similar solid objects) using three purifying stones. One should ignore the waswasah. Istijmar purifies the area, and therefore, semen discharged afterwards does not defile what it touches.

However, if istinja' with water or istijmar is not performed, and semen touches some urine on the area, it becomes defiled, and istinja' from it is required. Al-Baghawi reported that human semen is pure, but it becomes impure by coming into contact with urine or madhy (pre-seminal fluid) before washing the male organ. Similarly, it is mentioned in Sharh al-Muntaha that semen is pure, but if there is impurity on the exit point, then the semen becomes impure.

Finally, the jurisprudential schools confirm that semen becomes impure upon contact with impurity if the area is already defiled. The summary is that istinja' must be performed normally, without paying attention to waswasah.