Why does the Torah juxtapose a Nidah ("veha'Davah") next to a Zav?
Rashi (Shabbos, 83b): To teach us that, like a Zav, a Nidah is Metamei be'Masa someone who carries her.
What exactly is the Torah teaching us here?
Seforno (32,33): To remind ... the Zav - that he brings a Chatas [and an Olah] because he sinned both in deed and in thought; 1 ... the Ba'al-Keri - that he is Tamei because he did not intend for having children 2 ; ... the Nidah - that Nidus is a punishment for a sin; 3 ... the Zav - to remind him of a prior sin. The man and the woman should ponder the difference between their emissions, 4 and to point out that a man who has relations with a Teme'ah, adopts her Tum'ah, whereas a woman who has relations with a Tamei does not.
Targum Yonasan: It is reminding all the above-mentioned Teme'im to adhere to the laws that pertain to them and to bring the appropriate Korbanos so as to attain atonement.
What do we learn from "veha'Davah b'Nidasah"?
Moshav Zekenim (from Shabbos 64b): A Nidah is distanced; she may not paint her eyes or apply rouge. R. Akiva says, if so she will become repulsive to her husband 1 ! Rather, these are permitted, "veha'Davah b'Nidasah" teaches that she is Nidah until she immerses.
Moshav Zekenim, citing Avos d'R. Nasan (2): Her husband may not sleep with her in the same bed, even clothed.
Moshav Zekenim: If the Torah forbids, we must follow it! Rather, the verse is a mere Asmachta; R. Akiva argues with Chachamim who forbid mid'Rabanan, and the Halachah follows him.