"I say in all earnestness that it is a shame for parents to bring up their girls in such dangerous ignorance of the gins and nets that the wicked may set for them, whether their motive be a good one or the result of simple indifference." (46.14)
Here, Hardy voices his argument for simple sex education through the converted Alec D'Urberville – hardly a likely mouthpiece for the opinions of the author, but still, it's hard not to think that this is an opinion held by Hardy himself. Tess expresses the same frustration about her lack of education to her mother back in 12.81 – if she had known about sex, she would have been better able to defend herself against Alec's advances. This is a controversial opinion, though – in the late 19th century, it wasn't just a debate about whether or not to teach children about contraception as opposed to "abstinence only" – the general practice was not to explain sex to girls at all. People thought that the mere knowledge of sex would somehow make girls less pure and virginal. Through this whole novel, Hardy seems to be arguing that such a practice is short-sighted.