Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.7.9

4.6.7.9

tadvā etatsadaḥ pariśrayanti | etasmai mithunāya tira ivedam mithunaṃ caryātā iti vyṛddhaṃ vā etanmithunaṃ yadanyaḥ paśyati tasmādyadyapi jāyāpatī mithunaṃ carantau paśyanti vyeva dravata āga eva kurvāte tasmādadvāreṇa sadaḥ prekṣamāṇam brūyānmā prekṣathā iti yathā ha mithunaṃ caryamāṇam paśyedevaṃ tatkāmaṃ dvāreṇa devakṛtaṃ hi dvāram

That same Sadas they enclose on all sides with a view to that generation, thinking, 'Quite secretly shall be carried on that generation!' for improper, indeed, is the generation which another sees hence even when a husband and wife are seen, while carrying on intercourse, they run away from each other, for they give offence. Therefore to any one looking into the Sadas, except through the door, let him say, 'Look not!' for it is as if he were seeing intercourse being carried on. Freely (one may look) through the door, for the door is made by the gods.