It sounds like you tolerate (or enjoy, or whatever word is appropriate) one form of nicotine, but you have expressed an intolerance towards another. And your dislike of the other form actually extends to disgust, and disappointment in a person.
Isn't this a bit like enjoying and routinely drinking one form of alcohol, like whiskey, but despising gin, and showing disgust for anyone who orders a gin and tonic and ordering the other person to only consume the alcohol of your choice?
How did you express your disgust to chewing tobacco? Did you pretty much order your fiance to never use it? Did you describe, in great detail, how much you hated it and what kind of person would use chewing tobacco? What is it that you hate? It can't be the smell, or the nicotine stains, since those are inherent in cigarettes.
Now, I realize that some people who chew tobacco have a habit of spitting it, sometimes loudly or messily, into a plastic bottle or onto a sidewalk. That's as difficult to tolerate as someone who would grind out their cigarette in your coffee cup or who would throw their cigarette butt into your rose garden. I can understand if you ask your fiance to refrain from chewing tobacco when you're out with him, if he is constantly spitting or filling a plastic bottle with brown spit. We had a neighbor who would carry her spit bottle with her everywhere, even into my living room, and I asked her (well, I told her) that I wouldn't have that sloshing brown bottle on my kitchen table or in my house, since she was only stopping by for a couple of minutes, usually to ask for a favor. What she did in her own home was her business. And since your fiance is employed, I'm assuming he doesn't spit all over his desk. I see no problem with asking him to refrain from using chewing tobacco when he's out with you, or when you two are watching a movie at home. Just smoke cigarettes if that's your choice.
It seems like integrity is an issue you both need to work on. You're not asking him to stop using nicotine because you both use it, but you're asking him to use it in a way you dictate, and then questioning his truthfulness when he violated your behavior standards. And he is desperate for nicotine at work, so he resorts to chewing tobacco but doesn't tell you due to disgust and disappointment.
So, either nicotine is on the table, or off the table. Good manners are important, so if spitting and slopping is an issue, its ok to ask him to be a little more civil when he's out in public. But that applies to cigarettes, too. No just throwing them in the street, out of the car, or grinding them out wherever seems convenient. Use an ashtray, or an appropriate receptacle.