Was he a smoker previously, before you met him, and he has lapsed back into addiction? It seems incredibly odd for an adult to just suddenly take up something which is so stigmatised, unhealthy and not to mention expensive.
YANBU to be disgusted by it, especially as it is something he has began after the two of you got together. You wouldn't be unreasonable to leave him for it, for the same reason. At the same time, whilst an unwise choice, it's currently not illegal so is therefore his choice to smoke, and he will likely be highly addicted by this point and will actually have to WANT to quit to find the will power to come off them.
I say the above as a smoker myself. Well, part time at this point.
As a compromise, what if he switched to vaping? If his desire is just for nicotine, it's really useful as a substitute and allows me to go about my day to day life without cigarettes. The only thing that keeps me buying the occasional pack when I'm away from home is the desire to on my part, as I already vape 95% of the time. My wife is particularly sensitive when it comes to smells, so I can't smoke within a day of coming home, but vapour doesn't bother her. Granted, I don't use a hand held dry ice machine and use a simple menthol flavour, certain vapes and liquids can create noticeable smells, especially the sickly sweet ones. Most of the "smokers" I know at work are on vapes full time now, mostly at the request of their families.
As to why he has started, it really can take just one to get you hooked. I know a guy who had given up for years, who was offered a cigar during a drunken evening consoling someone after a bereavement. That one night was enough for him to go back full time smoking for 18 months untill he got a handle on it again. Maybe look into why he has started.