We are really talking about two different things here. 1.) How do you present gaming when discussing it with non gamers, and 2.) Are you mindful of what you are discussing when talking to fellow gamers in public places. The first one is pretty easy to answer, but the second one (for me) is a 'shades of gray' sort of situation.
1.) When discussing gaming with others I do my best to portray it as an activity that fosters relationship building, problem solving, basic mathematics and reading comprehension; as well as an inspiration for creativity in a number of ways (I know people who write, draw/paint, write computer programs, and so forth due to their interest in gaming).
After I establish all of that I tailor additional information to the person I'm talking to. I never discuss dark or 'fringe' subjects with non-gamers. I'm not hiding anything. I'm not a dark gamer. Vampire and Werewolf were about as dark as I got, and even in those games I reserved the darker aspects for the antagonists. Quite frankly if I'm talking to someone who is going to get turned on by talks of demonic pacts or the slaughtering of innocents, I keep the conversation casual. I don't likely want to game with that person anyway...
2.) Sometimes your talking with fellow gamers and you get so caught up that you forget where you are and who is around you. I've been given dirty looks, handed religous fliers in resturaunts, and even had a teacher separate a group of us during lunchtime at school so we couldn't discuss gaming (a situation that was cleared up quickly through one of the other teachers who happened to be a gamer as well). I try my best not to discuss subjects that will disturb or overly annoy people around me... but sometimes you get caught up and forget yourself. In those cases you apologize to offended parties, or (if you feel up to it) try to explain what it is that your talking about.
I personally love a good debate, and the 'religion -vs- role playing' one is one of my favorites (I've gotten a lot of practice over the years when dealing with my very religous, and very sincere and caring (although horribly misinformed), mother and her attempts to save me from myself). So I'll welcome that discussion more often than I'll try to avoid it.
I have a tendency to speak in first person when talking about my characters "I did such-and-such" instead of "Kira did such-and-such". With SR, this can get interesting; if you haven't played the game, you play the bad guys, or at least the neutral ones. Your adventures are things like assasinations, thefts, etc. So more then once I've caught myself talking about shooting someone or whatever and realized I had to change it to "my character shot someone". Last time we had a random security guard sitting in the restaurant booth next to us. Thankfully, I live in NJ, and most people mind their own business. 
