I'm aware I'm a bit late on this, but Cameron is looking at all-woman shortlists, imposed from the centre. This is not a conservative response to a problem!
Yes, there aren't that many women in Parliament. There are some very good women (Ann Widdecomme comes to mind) who got there by merit. The only female prime minister, Margaret Thatcher, got there by merit. That is the conservative way - the best person for the job gets it - an extension of Smith's division of labour surely.
That principle should be applied here. We want the best MPs, not the best female MPs, because they won't necessarily be the best MPs (they might, but it's not as likely). As well as this, isn't it quite insulting to women that they have to be given favours to gain seats over men?
ConservativeHome sums it up very well for me. It also mentions the central imposition of shortlists - it is up for the local association to choose a shortlist, and the candidate should be chosen in an open primary.