I think it can depend on a case by case situation, and of course how confident you are in your orientation (as well as time, how much you need to explain how much energy you have). I always have certain go to explanations for example getting them to imagine how they'd be in a romantic relationship with the sex or gender they aren't attracted to (it's amazing how many times that doesn't work, esp with cishet men), or the positive approach of getting them to think back to their best friends or besties before romantic relationships were prioritised in their life. Again, you can deal with idiots, but my experience is that it can take more than one chat to explain to the average person. I have the attitude that it's no secret I'm aromantic asexual but I don't speak to strangers about it and only personally bring it up for a reason, and then I always like to make people feel they can ask sincere questions. It is a difficult issue though, and LGBT groups do need to be educated primarily in some ways. There is no overnight solution.