Are damp spots on clothes or the ground, the impurity of which is doubtful, considered impure? And is the stain that resembles the color of blood or urine judged to be impure? And is it obligatory to change one's underwear immediately upon the penis touching it?

The default state of things is purity, and nothing is judged to be impure except with certainty. Therefore, anything whose impurity is doubted—such as moisture on clothes, wetness on the ground, or red or yellow objects—remains judged as pure. Similarly, the sticking of the penis to clothes does not render them impure or necessitate changing them unless you are certain of the presence of impurity. It is advised to disregard these obsessive thoughts.