Shatapatha Brahmana 1.9.2.5

1.9.2.5

atha patnīḥ saṃyājayanti | yajñādvai prajāḥ prajāyante yajñātprajāyamānā mithunātprajāyante mithunātprajāyamānā antato yajñasya prajāyante tadenā etadantato yajñasya mithunātprajananātprajanayati tasmānmithunātprajananādantato yajñasyemāḥ prajāḥ prajāyante tasmātpatnīḥ saṃyājayanti

They now perform the patnîsamyâgas. From the sacrifice offspring is assuredly produced; and (that offspring) produced from the sacrifice is produced from union; and (the offspring) produced from union is produced after (in consequence of) the completion of the sacrifice: hence one thereby (i.e. by the patnîsamyâgas) causes that (offspring) to be produced by means of a productive union after the completion of the sacrifice. And so now also offspring is produced by means of a productive union after the completion of the sacrifice. This is why they now perform the patnîsamyâgas.