Charak Samhita
बीजं यस्माद्व्यवाये तु हर्षयोनिसमुत्थितम् । शुक्रं पौरुषमित्युक्तं तस्माद्वक्ष्यामि तच्छृणु ॥१३३॥
The reason behind calling it as seed will be explained by me . The seed as an effect of delighted coitus, has been described as shukra,it is the male characteristic known as semen.
english translation
bIjaM yasmAdvyavAye tu harSayonisamutthitam । zukraM pauruSamityuktaM tasmAdvakSyAmi tacchRNu ॥133॥
hk transliteration by Sanscript