Amr bin Maymun (may Allah have mercy on him) said: I asked Sulayman bin Yasar (may Allah have mercy on him) about semen that gets onto clothes: should it be washed, or the clothes? He said: Aishah (may Allah be pleased with her) said that the Prophet (peace and blessings be upon him) would wash the semen and go out to the prayer in those same clothes, and I would see the marks of washing on his clothes.
The context of the hadith of Muhammad bin Bishr indicates that the sweat of a junub person is pure, and that it is not obligatory to wash the clothes in which he became junub; the semen is washed only for cleanliness, just as spittle and the like are washed from clothes for cleanliness, not because of impurity.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4181