"[...]I think the next thing I will need to come clean about at least to my partner is my recent experimentation with smoking. Actually- I think I would find it more enjoyable if I knew I wasn't doing it behind his back. I have no problems deceiving the rest of the world though... "
I think that there is obviously a huge difference between being a closet smoker with those whom you don't have an intimate relationship, and with those whom you do, in a way that is exactly analogous to whom it is appropriate (and IMHO, essential) to reveal a SF. And closet smoking in the absence of a SF is very different than closet smoking with a SF, especially when it comes to those with whom we are intimate, because it isn't just smoking for the sake of smoking - it's actually sexual.
Up to a point, being a closet smoker, and having a private kink like a SF can be a lot of fun in the sense that few things can be more exciting than having a Secret. But keeping such a huge Secret from an intimate partner is ultimately very, very maladaptive and harmful to both the person with the Secret, and the person from whom the Secret is being kept.
"I think that I would be truly happy if my partner said- go ahead and smoke whenever you want to. Then at least I wouldn't feel the need to hide those desires from him... But he, like most in a committed relationship, wants to keep me around for as long as possible and sees smoking as barrier to that since it is dangerous, unhealthy... etc.
"I feel like this is a bit of an impasse, but I plan on talking to him about it more ..."
Based on your comments about what you've shared with him thus far, how he has reacted, and your impression that he doesn't quite yet completely understand the seriousness of your attraction to smoking, it sounds like one avenue towards helping him to understand just how significant your SF is would be to simply tell him that you've smoked on several occasions, and that you plan to do it again.
By doing this, you will provide an invitation to further discussion, you will avoid any feelings of guilt over not being honest with him, and you will move the process forward toward integration and acceptance of all aspects of your SF, including your smoking, into the relationship, or you will both decide to reject the relationship and move on.
Look at it this way - if the latter happens, would you rather have it happen now, or many years from now? What would be less traumatic? What would be the most fair to you and to him?
"...since I think he equates my occasional smoking with me eventually becoming a smoker, which could happen- I cannot deny the addictive properties of nicotine, although part of me does every time I have a cigarette."
CF, you already *are* a smoker!
Even if you only smoke one cigarette per month from this point forward, you're still a smoker, and based on everything you've said, it sounds to me like it would be absolute torture for you to not allow yourself the sexual pleasure that smoking provides your SF, on at least a semi-regular basis.
I suspect that you are referring to becoming a daily smoker of at least several cigarettes, but it seems pretty clear that the option of you never smoking again is extremely remote, so if either you or your partner are afraid of you reaching even a modest level of daily consumption, this issue will torture you both endlessly. Perhaps it might be best to simply confront this fear head on, and to recognize that whoever you are with needs to be O.K. with the possibility of you becoming a daily smoker, even if it never happens. Neither of you needs to have the pressure of worrying about this constantly, don't you think?
As for nicotine alone being responsible for addiction...
Once again, I believe that nicotine is a significant, but is no way the most significant aspect of addiction to cigarettes. Addiction to nicotine isn't that much stronger than addiction to caffeine is, from a purely pharmacological point of view. As I've discussed here many, many, many times before, I am absolutely convinced that the real strength of addiction to cigarettes derives from the psychological and sensual aspects of the act separate from all pharmacological considerations, as well as the psychological and sensual aspects in response to the physical manifestations of nicotine addiction.
Said another way, I believe that addiction to cigarettes is not an inescapable outcome of being exposed to X amount of nicotine over Y amount of time. There is no "Zombie" effect of nicotine exposure, and if you ever did lose your desire to smoke from a psychological point of view, I'm convinced that you absolutely could overcome the lesser pharmacological element and quit and never smoke again if you really, really wanted to completely stop.