You don't need to offer explanations. But what I am hearing is that maybe you don't feel comfortable not drinking and being at a party where everyone else is drinking. This is understandable. I used to go to big bash parties in graduate school even though I wasn't drinking.

Everyone wanted me to attend and they were very kind and always provided me with my favorite soft drink. But in the end it really wasn't that much fun because people did drink heavily at these parties.

What I finally did was this....I would make an early appearance while people were still snacking on food more than drinking. I would eat some snacks and have a soft drink and socialize. Then I would leave before the heavy partying started. This worked well. I can understand your apprehension as I had it too. But you would actually be surprised how little people really care. If you make an appearance you won't be rude...but you don't have to stay so long that you start to feel uncomfortable.