Usual smoker argument to quote the harm drink does when we're not even talking about drinking. My father's a bloody alcoholic (so I know all about that thanks) but aren't we talking about smoking and how smoking affects the choice of non-smokers not smokers.
Went to the docs the other day with my 4 month old son who had a cough. First question was "Do either of you smoke?" Richard and I were mortified that he would think that we could poison our son's lungs by smoking. But people do it, and children have no choice. How many people do you see pushing buggies with a fag in their hand, smoke drifting down into their child's lungs? The other day I went to the cash machine and a woman was there smoking,blowing the smoke into her child's face, I went to use the machine after her and she'd spilt fag ash all over the keypad...lovely.
So this is the habit you want to protect? Fair enough-enjoy it while you still have the chance.