i think its a very difficult issue. while a 13 year old is legally (and very very likely still mentally) very much a child, they are not the same creature as say, a difficult 7 year old, and you can't really effectively handle them the same way.
a willful 13 year old will find a way to have sex, if they are dead set on doing it, even if you try to watch them every minute, no matter how much you try and educate them, rationalise with them, explain that its an incredibly bad idea, explain to them the repercussions of their actions, forbid them from doing it, threaten them, etc. if they are going to do it, they will find a way, unless you chain them 24/7 to a radiator in a locked room- and thats child abuse.
i agree that parents should be at the forefront of dealing with this issue. its vital that its acknowledged that they are the parents, she's their child, and that they have beliefs, ideals, a moral standpoint, feelings, expectations, and that if possible, they have a big say in what happens. but its also important to consider that a child who is having sex is at risk of experiencing a lot of things which could have a very negative impact on their life- including std's, HIV, teen pregancy (i know many people have a positive experience of teen pregancy, but i don't think a 13 year old is in a position to have the emotional maturity to rationally decide to become a mother, and the skills to support herself emotionally or financially, and think it should definately be avoided).
in my mind, the welfare of the person who is having sex and is at risk of harm (the child) comes before the welfare of the parents- who aren't at risk of harm in the same way. someone who is at risk of harm, and asks for help to avoid that harm, should be armed with tools which they will use to avoid harm coming to them, be they a child or adult. if they are going to have sex, no matter what you try to do to stop them, then they should be provided with a way to try and protect themselves while having sex.
i don't think that just handing them the pill is the anywhere near the best option to use for this though- kids in the uk who approach services in confidence for help, tend to be assessed in detail, and if appropriate, are given a wealth of information, someone to talk to about issues, and to make sure they understand what they are dealing with, and generally recieve the contraceptive shot from a doctor, and are given and taught how to use free condoms.
in my area, we actually have a new 'sex card' system running, which is given to older kids who are having underage sex, after they have completed a course which ensures they are properly educated in all the repercussions of their actions and how to use the tools that they are given. if they present this card at a range of locations and centres, they can receive free condoms and sex related info without getting any [email protected]
or a million questions about it- because its been found that if you give young people a huge earful, or try and stop them having sex, they'll just leave, and have sex anyway- often unprotected- which is a [email protected]
sight worse in my books.
if your kid has cancer, or poor nutrition, or another medical or health issue, and as a parent you refuse to treat him, or choose to treat him in a way which isn't deemed to be effective, doctors can overrule you, and provide what they deem to be the best treatment, because the law says its in the best interests of the child- providing a child who is at risk of harm through sex with practical tools, information, and contraception, similarly seems the logical thing to do, and in the best interests of that child, to me.