The Mukhya Upaniṣads

Aitareya · Adhyāya 2 · Khaṇḍa 1 · The Three Births

Passage 2

तत्स्त्रिया आत्मभूयं गच्छति यथा स्वमङ्गं तथा ।तस्मादेनां न हिनस्ति सास्यैतमात्मानमत्र गतं भावयति ॥ २ ॥

In plain words

That becomes one with the woman's own self, as if it were her own limb; so it is. Therefore it does not harm her. She nourishes this self of his that has come there.