Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.20

1.1.1.20

tā uttareṇāhavanīyam praṇayati | yoṣā vā āpo vṛbṣāgnirmithunamevaitatprajananaṃ kriyata evamiva hi mithunaṃ kLptamuttarato hi strī pumāṃsamupaśete

He then carries it north of the Âhavanîya fire. For water is female and fire is male: hence a copulation productive of offspring is thereby effected. And in this way alone a regular copulation can take place, since the woman lies on the left (or north) side of the man.