I don't lie about my reasons for eschewing certain foods, but that doesn't mean I'm always forthcoming about being "paleo" or "primal" or whatever I am. I can think of a hundred different ways to address this situation--some of which others have already suggested here--without needing to lie, nor to reveal my personal issues. Personally, I don't want the tangled web someone else suggested, whereby one sets one's self up for exposure later when one decides to try a treat meal.
"I've made some big changes to my diet, and so far it seems to really be helping. I can't say I'll never eat hot dog-mayo sushi (really??) again, but I'd really like to stick with this for awhile to see how it goes."
Later I'm spotted in a moment of weakness, judgement clouded by alcohol, ordering the hot dog-mayo sushi. "It's been months since I've had it--I thought I'd try it again to see what happens. I may never touch it again...."
Having said that, I find the lectures about lying, and particulalry this notion of the "disservice" done to people with celiac disease, et al., rather a stretch. We don't owe anyone else this supposed service. And it's not my job to use my dietary preferences in some ambassador role, nor to function as some sort of role model for healthy eating. If that's what people get from me when they see my improved health, and strange diet, great! And if they ask me, I'm all too happy to spread the word. But I don't owe them that. This isn't the same thing as Paula Deen, who lied for years about her health in order to protect a commercial enterprise, and then used that situation to further enrich herself by profitting from the sale of the drugs treating her condition.
Here, we're discussing a white lie, told in part to spare another person needless guilt or shame. This does no "disservice" to people with true celiac disease, et al.--it's not even about those people. It's between two people negotiating their lunch. The OP's white lie in no way precludes true celiacs from, upon meeting someone offering food, truthfully claiming their legitimate conditions. This seems a classic example of making a mountain of a molehill.
Philosophers and clerics will debate lies forever, but contemporary society seems to fully embrace the white lie told to avoid unnecessary and pointless pain, discomfort, or embarrassment.
"What do you think of my new jeans?"
[I think they make you look like too much sausage stuffed into too small a casing.]
"They're very stylish--are they Levis?"
In the OP's case, a small untruth, in a private setting, between two people for whom the basis of the untruth does not constitute a truly significant or necessary part of the relationship, seems compassionate to me. Something can be both compassionate and self-serving at the same time. I don't feel we can reasonably liken a white lie such as this to, say, marital (or romantic) infidelity. Anyone wagging that accusatory finger will have a difficult time claiming they themselves never lie.
Who among us hasn't excused ourselves from a conversation with the claim that we are busy or running late, or something similar? When we do so, do we do a disservice to truly busy people? Or do most of us agree that this lie seems preferable to saying, "I'm tired of talking to you now?"