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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
मेढ्रं तस्य विनिर्भिन्नं स्वधिष्ण्यं क उपाविशत् । रेतसांशेन येनासावानन्दं प्रतिपद्यते
meḍhraṃ tasya vinirbhinnaṃ svadhiṣṇyaṃ ka upāviśat | retasāṃśena yenāsāvānandaṃ pratipadyate
SB 3.6.19