Markandeyapurana 49.10

49.10

āyuṣo 'nte prasūyante mithunānyeva tāḥ sakṛt /
tataḥ prabhṛti kalpe 'smin mithunānāṃ hi sambhavaḥ //

Then those living beings, produced in pairs, were rejoicing together; urged by their mutual distress they hastened to sexual intercourse. Thenceforward pairing originated in this kalpa. Women did not have their courses month by month; hence they did not then bring forth offspring, although they engaged in sexual intercourse. They bring forth just pairs of children once at the close of life. Thenceforward pairing originated in this kalpa. By meditation and thought those human beings give birth to offspring once. Sound and the other objects of sense were pure severally in their five marks.