My answers:
In the firearms and contraception examples I think it's appropriate to downvote if the question is posited in such a way that it presumes it is a good idea if you don't agree that it is a good idea. Somebody is asking how to do something when you think doing it is harmful or evil. Since it's subjective, the question is OK, but I think it's OK to downvote.
In the HIV example: Vote to close and explain. Such a question is beyond help, and most likely so is the poster. (Update: In the case of less paranoid factual inaccuracies see next point.)
In the Tom Sawyer example: Edit and comment. It's just a factual mistake.
In the circumcision example: Do nothing. It's not a question if you should circumcise or not, it's already happened. Any opinions on that is too late anyway.
In the sleeping example: Do nothing. Nobody that advocates co-sleeping claims it's for ever or for everyone. You shouldn't have to explain yourself for why it's not working for you. I've encountered that attitude twice now, and it's annoying. It's my life, my kids and I know that I have no choice in the issues. Being questioned isn't helpful.
Update: Some notes after reading HedgeMages answers: The HIV question would likely be asked by a conspiracy theorist, and they are not receptive to rational argumentation. From HedgeMages answer I realize that is perhaps not what the question was intended to be an example of.
Also, before somebody accuses me of bias or other straw men (which I've unfortunately come to expect of this site already) I disagree with introducing kids to weapon and agree with sexual education.