Well, I think the only physics involved is whether or not you bring that drink to your mouth or not.
But I do understand the hesitation about making a promise that you worry that you cannot keep, especially when it comes to others. What happens if I fail that commitment, is it a moral transgression? Do I evaluate myself as a failure or the idea of NEVER doing something again as unreasonable? I have some trouble with wholly black and white standards, but not all, and is this one of those things that is a hard line that I will not cross?
Maybe some day I will be able to assuredly state that I will never drink again. Today I'm sure I'm not going to drink. I have not drinking ever again as my ideal that I strive toward, and it's a pretty simple one (as has been said here) to follow. I just will not drink. Somebody recently wrote that it was an achievement to say now with confidence, "I don't drink" from a place of "I cannot drink". That's where I'm aspiring to be.
So, in that sense it does become a moral choice for the day, for me, as it affirms values that I esteem. Our reality is not just the physical laws that we cannot break, like getting sick after drinking does to me, it is also our perception and our actions that form a basis for living in a way that reality doesn't bite.
I'm rambling a bit, sorry, but I'm still working through a lot myself.