they're teenagers. their brains are not fully formed yet and they will do and say stupid shit all the time.
Best advice is to maintain your cool, acknowledge their ability and right to think for themselves, while setting strong boundaries for your own beliefs (ie, while you may not agree with me, you may not disrespect me/talk to me like that). Once they're out of the dumb teen phase, they will respect you even more. Just a long couple of years until they're out of it.