Back to everyone's corners with the sniping, maybe?
Trying- everyone is not on a high horse- we just all have our opinions or situations and some of us are more adamant about some of the scenarios we face as alcoholics.
I, for one, think white lies are ok in instances where it fits. That's your judgmental call. The solution you used isn't a bad one per se; for me, it would make me think of times I did the reverse- ordered club soda and lime and went to tell the waiter I wanted vodka soda.
One thought that occurred to me is that you are making (perhaps, just my read here) a lot of assumptions about what your uncle wouldn't accept- ie, a change in your behavior. And, perhaps, it is not as important to him to have a drink with you as you think - rather, your company is more important, since you obviously have a close relationship.
I tend to go the "I don't drink" route of some version- and deftly change the subject. I AM of the opinion that we don't owe ANYONE an explanation or reason, though it is often natural to feel that we do.
I also do not spend time with ANYONE who wouldn't/doesn't accept that I don't drink. Fortunately, I don't have any family I have to worry about on that end- and I would frankly be put off by THEM if they had a problem with me not drinking.
Ultimately, you decide what to do.
To your final point about black and white....well, drinking or not is just that for me and for a lot of us. If it is that simple, there is no grey. It's just how you handle a life or death, black and white issue.