I know what you mean. We are also planning a UC (anytime now!!!) and most of the family is unsupportive and most of my friends/acquaintences would have a coronary if they knew. And I don't lie. I just don't--I'm horrible at it, I feel horrible doing it, and I think it's wrong. But I also think it's wrong to expose myself to other people's fears and negativity. I wouldn't expose myself to things I know are physically toxic while pregnant, and I won't do the same with emotional/mental toxic thoughts and attitudes.
Luckily, most people haven't asked much until now, when I'm obviously very close. So, I've started telling people that we aren't discussing it. I explain a bit about how I don't want my preconceptions and any possible fears to get in the way of the birth, so we've chosen not to discuss the details of the birth for these last few weeks. And I ususally add that I'll give a full report after the fact. People are generally responsive to this--surprisingly. I make it sound like I understand they are trying to be supportive but that right now, my needs are coming first and I need to not discuss this.
Also, I think most people have a pretty fearful understanding of birth and they somehow feel like they are helping me by not scaring me. And I think most people need to be educated about what topics are appropriate--I think it's the height of rudeness to be telling a pregnant lady about how horrible birth is--we should not have to hear about their misconceptions.
Immediate family and my closest friends do know about our plans. I'm sure there will be some people who will be surprised and a bit shocked when they find out the details later and realize we never told. But I'll deal with that then. I'm not lying now, I'm not even evading--I'm telling them I'm not telling them and my reasons for not telling them are truthful.
Hope you find a way that works for you. Violet