Dont be a little shit. Your parents are just worried and trying to protect you. They might be ignorant but their intentions are good. You need to show maturity and responsibility, handling your shit right and being on top of things. And then you can talk to them like an adult and propose to explain how you feel about it, scientific facts and so on. Ask them honestly what they know, if they have or have ever had any friends who smoked and discuss their feelings about it. Say you have friends or know people whose relationships with their parents turned really bad because of issues like this and that you would hate to distance yourself from them. That they both know you will move out eventually and that you want to maintain a positive relationship. You can do whatever you want in the future when you're away and they can't really stop you, so the least they can do is agree to sit down with you and research about it, talk to doctors and people who have experience in the area.
If you are worried about the approach, just come to the parent that is most receptive to it, or at least who you have an easier time dealing with, and say that since you are responsible, have good grades and are trying hard to do everything right you feel you deserve a little bit of their time to talk about this. Say you will respect their conclusions and opinions no matter what, but it would mean a lot to you if they would at least listen to it and talk to professionals who understand more about drugs. You can say that you respect their views but you feel it would be hard to communicate directly since they as parents are trying to protect you and do what they believe is best for you, and you have your own interests which are different, so you would both benefit greatly from having a professional or an outside party who they respect intermediating the situation. It's best to talk to the good cop individually and let them work on the bad cop in private later on.
Make sure they understand how good this move on your part is because honestly 99% of young guys would just ignore their parents and keep doing it behind their backs. But since you respect them too much you would never do that and want to have an honest heart to heart talk about it. If they are really being super difficult make sure they know you are not trying to get them to agree to your drug use by any means, but that you feel everyone can only benefit from more information on the subject even if their ultimate stance doesn't change. Even if they still feel the same way in the end, by understanding better what the drug actually does and doesn't, how long the effects last, the different means of consumption and so forth they will be in a better position to deal with it in the future anyhow.
Now since they are bible thumpers this will in all likelihood not change their minds, but if you can through to them that it is not as serious an issue as they imagine it to be, and that you have a responsbile harm-reduction approach to it (you wouldn't smoke anything that is adulterated or low quality, you wouldn't smoke before a test or while studying) this might take a huge load off their minds and make them view your drug use in a different light in the future.