Charak Samhita
तत् स्त्रीपुरुषसंयोगे चेष्टासङ्कल्पपीडनात्। शुक्रं प्रच्यवते स्थानाज्जलमार्द्रात् पटादिव ॥४७॥
The semen trickles down from its site during copulation between man and woman because of sex act, passionate attachment, and physical pressure, as water comes out of a wet cloth when squeezed.
english translation
tat strIpuruSasaMyoge ceSTAsaGkalpapIDanAt। zukraM pracyavate sthAnAjjalamArdrAt paTAdiva ॥47॥
hk transliteration by Sanscript