SB 3.12.55

tadā mithuna-dharmeṇa
 prajā hy edhām babhūvire
Synonyms: 
tadā — at that time; mithuna — sex life; dharmeṇa — according to regulative principles; prajāḥ — generations; hi — certainly; edhām — increased; babhūvire — took place.
Translation: 
Thereafter, by sex indulgence, they gradually increased generations of population one after another.