Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said regarding semen that gets onto clothes: Remove it from yourself. One of the narrators said: Clean it with a stick of idhkhir, for semen is like spittle or mucus.
This is authentic from Ibn Abbas as his own saying; it has also been narrated as marfu' (from the Prophet), but its being marfu' is not authentic.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4175