OK I'll bite. I haven't read the book, and its not the world's best written book review but...
Onehandflapping - it doesn't ask why should women have sex, but why dothey (or at least why do they report they do in whatever the survey was) which seems like a legitimate and interesting question.
Ladyblablah - it wouldn't have been much of a survey if it was only allowed to give one answer
...and anyway, if the only reason is orgasm, there are easier and simpler ways to achieve that.
I think what they did was do a survey which asked a bunch of women 'have you had sex in the last ....' and then 'why did you have sex'....so if some of those women answered 'because I was raped/cooerced' then that is their answer, no? I don't see why it should be ruled out.
....I did think the bit where they said men often have sex on the first date, but women often have sex on the third date a bit
...somebody must be embroidering the truth in their survey responses there surely?!