וְאִ֡ם שָׁכֹב֩ יִשְׁכַּ֨ב אִ֜ישׁ אֹתָ֗הּ וּתְהִ֤י נִדָּתָהּ֙ עָלָ֔יו וְטָמֵ֖א שִׁבְעַ֣ת יָמִ֑ים וְכָל־הַמִּשְׁכָּ֛ב אֲשֶׁר־יִשְׁכַּ֥ב עָלָ֖יו יִטְמָֽא׃ (פ)
At kung sumiping sa kanya ang isang lalaki, at mapasakanya ang kanyang karumihan, magiging marumi siya nang pitong araw; at bawat higaan na hinihigaan niya ay magiging marumi.
Rashi
ותהי נדתה עליו [AND IF ANY MAN LIE WITH HER AT ALL] THEN SHALL HER UNCLEANLINESS OF SEPARATION BE UPON HIM — One might think that he follows in her wake — that if he had intercourse with her e. g. on the fifth day of her separation he shall become unclean for three days only (for the fifth, sixth and seventh) like herself (i. e. because these are the remaining days during which she is unclean)! Scripture, however, states: "he shall be unclean seven days”. Therefore these words cannot refer to the duration of her uncleanness. What then is the force of ותהי נדתה עליו? They mean: her state of uncleanness as a נדה shall be upon him. How is it in the case of her? She, being an אב הטומאה, makes man unclean, and also earthen vessels by means of היסט! So, too, he becomes an אב הטומאה (not a ולד הטומאה), and makes man unclean, and also earthen vessels by means of היסט! (Niddah 33a)
Ibn Ezra
if a man has sexual intercourse with her unintentionally (for example, if she began to menstruate while she was with him. It is in this sense that we understand the word upon him ), since karet applies to the man who intentionally sleeps with a menstruant. The same law applies to a woman with a constant flow, although it is not explicitly mentioned.
Rashbam
אשר ישכב עליו יטמא. This degree of ritual contamination is sufficient to confer further contamination on food and drink coming into contact with it. The severity of contamination of someone lying on top of a menstruating woman is equivalent to that of someone below a male afflicted with the disease of being zav. This is spelled out in Niddah 32.