Śatapatha Brāhmaṇa ("of a hundred paths") 6.4.4.19

6.4.4.19

pariśritam bhavati | etadvai devā abibhayuryadvai na imamiha rakṣāṃsi nāṣṭrā na hanyuriti tasmā etām puram paryaśrayaṃstathaivāsmā ayametām puram pariśrayatyatho yonirvā iyaṃ reta idaṃ tira iva vai yonau retaḥ sicyate yonirūpametatkriyate tasmādapi svayā jāyayā tira ivaiva cicariṣati

It is enclosed on all sides; for at that time the gods were afraid, thinking, 'We hope the Rakshas, the fiends, will not smite here this (Agni) of ours!' They enclosed him with this stronghold; and in like manner does this one now enclose him with this stronghold. And, again, this is a womb; and this (clay) is seed; and in secret, as it were, the seed is infused into the womb: it is thus made of the form of the womb; and hence it is only in secret that one would have intercourse even with his own wife.