SB 3.6.19
medhram tasya vinirbhinnam
sva-dhisnyam ka upavisat
retasamsena yenasav
anandam pratipadyate
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the genitals of the gigantic form separately became manifest, then Prajapati, the original living creature, entered into them with his partial semen, and thus the living entities can enjoy sex pleasure.