Why does the Torah add the prefix 'Vav' in "ve'Ishah ... "?
Nidah, 32a: To incorporate a girl of three in the current Halachah. 1
Since, before that, she is not capable of intimacy. See Torah Temimah, note 65.
What are the implications of the word "asher Yishkav Ish osah"?
Sifra: It precludes a Katan (under nine) 1
Refer to 15:16:0.1:1*
What is the word "asher Yishkav Ish osah" coming to preclude?
Yevamos, 34b: It precludes the woman 1 from Tum'ah in a case where they had unnatural intimacy.
The man however, is Tamei (Torah Temimah).
Why does the Torah insert the (otherwise superfluous) words "Shichvas Zera"?
Shabbos, 86b: To render Tahor Zera that has turned putrid.
Yevamos, 34b: To us that a if a man performs Ha'ara'ah (sexual contact without penetration 1 ) they are not Tamei.
Which does not entail the emission of Zera (Torah Temimah).
Why is also the woman Tamei?
Rashi: She is Tamei due a Gezeiras ha'Kasuv. It is not because she touched the Keri, since the touching took place internally (Tum'as Beis ha'Setarim). 1
And Tum'as Beis ha'Setarim is not Metamei (Sifsei Chachamim).
Why does the Torah write "ve'Ishah, ki Yishkav Ish osah", and not 've'Ish ki Yishkav es Ishah"?
Seforno: To teach us that a woman is Tamei either by seeing blood or by exuding her husband's Zera, 1 (in spite of the fact that it is Tum'as Beis ha'Setarim).
If her body would produce Zera, it would not be Tamei. A man's Zera is Metamei her internally (via Bi'ah), and externally, including when it leaves her body afterwards. (PF)
Why did the Torah not mention that they must wash their clothes?
Moshav Zekenim: This hints at Chazal, who say that, if a man insists that they have relations while dressed, [if she objects] he must divorce her and pay her Kesuvah
