What is the ruling on sleeping on sheets and blankets where there is doubt about madhy having fallen on them, and what should be done about that?

The default state of things is purity, so do not pay attention to doubt and rule the bedding as pure. Semen is pure, according to the preponderant opinion, though it is better to wash what it has stained. Madhy (prostatic fluid) is impure, and for how to purify it from bedding, refer to Fatwa No. 147825.