"No", by itself, isn't really an answer here, as funny as the thought is. Realistically, "Should I grab a condom?" is proposed in a time of physical intimacy. That question would be followed with one of three responses meaning, "Yes," "We don't need one," or "We are not having sex."

Of course, "No," is a possible answer, I'm just trying to imagine a situation in which one person is ready to have sex but is unsure if the other person is or not and they actually lack physical or intimate context...

saying nothing other than "No" to that question would be quite rude.