Why is also the woman Tamei?
Rashi: She is Tamei due a Gezeiras ha'Kasuv. It is not due to touching the semen, for it did not touch her externally. 1
Sifsei Chachamim: Tum'as Beis ha'Setarim is not Metamei.
Why does the Torah begin with "v'Ishah ... ", and not with the words 'v'Ish ki Yishkav Es Ishah ... "?
Seforno: To teach us that a woman is Metamei either by seeing blood or by seeing Zera, but that the latter is only Tamei via Bi'ah with a man 1 - despite the fact that it is Tum'as Beis ha'Setarim.
I.e. 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 drips out of her afterwards. (PF)
Why did the Torah not mention that they must wash their clothes?
Moshav Zekenim: This hints to what Chachamim said, that if a man insists that they have relations while dressed, [if she objects] he must divorce her and pay her Kesuvah. (I.e. they are naked at the time.)