Abdullah bin Abbas (may Allah be pleased with them both) regarding semen that gets on clothes, said: "Remove it with a piece of wood or grass, because it is like saliva or nasal discharge."
Hadith Referenceمسند الإمام الشافعي / كتاب الطهارة / 26
Hadith Takhrijصحيح موقوفاً: أخرجه ابن أبي شيبة (924) - والدارقطنی: 1/ 125.