Kamasutra 1.2.19

1.2.19

sā copāyapratipattiḥ kāmasūtrād iti vātsyāyanaḥ//

This is not so. Sexual intercourse being a thing dependent on man and woman requires the application of proper means by them, and those means are to be learnt from the Kama Shastra. The non-application of proper means, which we see in the brute creation, is caused by their being unrestrained, and by the females among them only being fit for sexual intercourse at certain seasons and no more, and by their intercourse not being preceded by thought of any kind.