If najasa (such as sperm) is present on the clothing, is not washed and dries, and if it leaves no traces (e.g. smell, colour etc) is it still obligatory to wash it? and does one still have to make ghusl?
Sperm — or more accurately, semen (Ar. maniyy) — is pure in our school. Hence, it is not strictly necessary to wash it away before praying. The exiting of semen from the penis necessitates a purificatory bath (Ar. ghusl). (Anyone wishing to take extra precautions should note that semen is considered filthy in the Hanafi and Maliki schools.)
However, the fluid that sometimes comes out during sexual excitement (Ar. madhy) is filth. One is required to wash it away and perform wudu before praying. The purificatory bath is not necessary.
If something impure (such as madhy or urine) is present on clothing but leaves no traces of smell, colour, or taste, then it is still obligatory to wash it. One simply has to ensure that pure, unused water passes over the affected area.
Allah the Exalted knows best.
Metro Vancouver, Canada