Dinnertime Discussion

Every night after dinner we use a book called ONE YEAR of Family Devotions to share God's word with our kids. Some nights the devotions are awesome and the stories really drive the point home, while other times they are a bit cheesy. Last night's devotion brought on an unexpected discussion. The verses used were from I Cor.3 about not being wise according to the world's standards. The story was about a boy named Rob and his dad, sitting on their deck, lit by the glow of a bug zapper. Rob confesses to his dad that he has been smoking. He says he realized looking at the bug zapper that in the same way the bugs while attracted to the light were being drawn toward their demise, that he was attracted to things that were harmful to him. Neat little story - until we started to discuss it.
It just so happened that as we were starting this discussion, still sitting at the table full of dirty dishes, that our dining room was filling up with teenagers from our neighborhood. Two of them just happened to stop by as we were starting our devotion. Maybe a God thing?
For the sake of conversation, I threw out the question whether or not "smoking" itself was a sin. I had no answers, I wanted to know what everybody, including my husband was thinking. Some thoughts were that it was a personal conviction. We talked about how your body was the temple for the Holy Spirit - but then how would smoking cigarettes, something that is harmful with no benefit to your body be any different than say, eating margarine which is supposedly one molecule away from plastic, and also harmful with no benefit.
I am an ex-smoker. I used to have times when I first quit smoking that I was repulsed and craved a cigarette at the same time. Now I'm pretty much repulsed by the smell. When I stopped smoking, it was a personal conviction - I felt that God was asking me to lay them down - because they were my crutch. I shared this - a distinct reason that cigarettes becoming an idol, as I made smoking the place I turned when I was stressed or lonely or bored, was a sin for me. Or how disobeying your parents, if they have told you not to smoke is the sin of disobedience. But is smoking in and of itself a sin? This was what we tossed around for about half an hour.
I explained to the kids that on a non-spiritual level - just as a health issue, staying away from cigarettes was a very wise decision from a healthy living standpoint. I have to admit, it makes me a little uncomfortable as a mother not to give my teenagers pat answers for things like this. It seems easier and safer just to tell them that everything is a "sin" that I am personally convicted about and settle the issue, but I find that I am not teaching them to live in relationship with the Lord, listening to His voice and learning to heed the direction of the Holy Spirit in their own lives.
I don't have an answer as to whether or not smoking is a sin. For me it would be because I would be disobeying a specific conviction. As far as respecting our bodies as the temples of the Holy Spirit, what about poor posture, bad sleep habits, unhealthy diets, etc. etc. etc. Would we call these things sin as well?
What are your thoughts?