I don't want to go there.
What you did here is stop the conversation dead in its tracks. While that was your intention, did you consider what was supposed to happen after you stopped it?
Think from your conversational partner's point of view. You've clearly protested the conversation they wanted to have. How should they respond to that?
- Should they ask the same question in a softer manner?
- Do they need to acknowledge that you didn't want to answer?
- Did they offend you and are you waiting for an apology?
Whatever option they choose, it's always awkward. And the reason it's awkward is because you didn't offer an alternative way out. The only way to keep the conversation flowing is to acknowledge what you said, but any response to that will be of a dramatically different tone than the conversation they were trying to have.
There are ways to make it less awkward. In all cases, a key part of this is to not respond viscerally (as you said). The other party wasn't trying to corner or offend you, they most likely asked what they consider to be an innocent hypothetical question.
Firstly, you can provide enough information that the other party knows your stance and thus doesn't need to awkwardly guess at how they should continue the conversation.
I haven't really thought about it and I'd rather not. We'll cross that bridge if and when we get to it.
Based on the answer you gave, the other party now knows to not continue on the topic, but they now also know that they don't need to particularly respond to your response (you're not upset with them).
Don't try to sound short or annoyed because that again halts the conversation. If you notice a pause, you can change the topic yourself in order to save them from the awkwardness of not knowing how to respond.
Secondly, you can try to avoid the direct question by checking to see if the other party is actually trying to find out something innocent that you don't mind answering:
Why are you asking?
Either they clarify that they actually want to ask the question they were asking (at which point you refer to the previous suggestion), or they clarify that they wanted to ask about something else that doesn't bother you.
It also has the added benefit of getting them to re-evaluate their question, in case there were actually being inappropriate.
Thirdly, although this might not apply to this particular situation (due to the life and death nature of it), you can deflect with humor. Do keep in mind that this only works in the right kind of setting, but it can work well in the right setting.
I've reconsidered and am not adding examples of humorous deflections as the subject matter is sensitive to you and tone of voice doesn't quite carry in a written format anyway.