In this material world the point of attraction is stri and purusa, man and woman, male and female. There is an attraction, natural. So the man wants woman, woman wants man, because there is attraction. And when, by that attraction, the man and woman is united, then the result is the children. Pumsah striya mithuni-bhavam etam tayor mitho hrdaya-granthim ahuh [SB 5.5.8]. Then the attraction for this material world increases. When one is alone, he's not so much attached with the material world. But as soon as he unites with the other party, then he gets children, and the attraction increases.
Lecture on Srimad-Bhagavatam 2.1.4, Vrndavana, March 19, 1974