SB 3.6.19

meḍhraṁ tasya vinirbhinnaṁ
 sva-dhiṣṇyaṁ ka upāviśat
retasāṁśena yenāsāv
 ānandaṁ pratipadyate
Word for word: 
meḍhram — genitals; tasya — of the gigantic form; vinirbhinnam — being separated; sva-dhiṣṇyam — own position; kaḥ — Brahmā, the original living creature; upāviśat — entered; retasā aṁśena — with the part of the semen; yena — by which; asau — the living entity; ānandam — sex pleasure; pratipadyate — experiences.
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.