I am not promoting it. I am recognizing it as barrier to moving people away from using animals as food. If saying something is a reason that it is hard to convert the larger public, is the same as promoting it, I am not sure how you go about discussing the hurdles to achieving this goal. The old saying "it is an explanation, not an excuse".
Well if I take it as a serious point, I dont see still how its useful to bring it up. We can't change our natural impulses, only how we react to them. Following a vegan diet is no more challenging physically or mentally than managing a regular diet if you have the same goals.
Its akin to saying that a mans nature makes it difficult not to sexually assault women. While technically true, it has nothing to do with identifying problems and creating solutions.
I'm struggling to find any good reason to bring up natural instinct besides as an excuse.
Except that most people get a negative physical response from hurting other people, where as very few get one from eating meat, cheese, and eggs. You have reduced something very, very, complex to an absurd degree to make your argument, in order to say talking about it is just excusing behavior. So have fun with unrealistically simplified world. You won't be accomplishing your goal living there, though.
What you are describing with meat is what happens not long after eating too much. In fact, other than lactose intolerance, it is rare to have issues that are not due to eating too much. Sure, there is a disease you can get from ticks, that can make you unable to process a protein, found in all animal products. Friend of mine has it. However, rare. Far more people have type one diabetes precluding them from eating a bunch of vegan friendly food.
That vast majority of people have bad reactions to hurting others though.