Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 19
Sanskrit:
मेढ्रं तस्य विनिर्भिन्नं स्वधिष्ण्यं क उपाविशत् ।रेतसांशेन येनासावानन्दं प्रतिपद्यते ॥ १९ ॥
ITRANS:
meḍhraṁ tasya vinirbhinnaṁsva-dhiṣṇyaṁ ka upāviśatretasāṁśena yenāsāvānandaṁ pratipadyate
Translation:
When the genitals of the gigantic form separately became manifest, then Prajāpati, the original living creature, entered into them with his partial semen, and thus the living entities can enjoy sex pleasure.