I would leave ethics out of it all together.
Why not just say, "for personal reasons, I won't do ______"
Because I would think it would stop conversation. To me, ethics is debatable. My ethics aren't set in stone, and change. If someone just said "personal reasons" I would assume that meant "back off, I don't want to talk about it".
I think that if someone were to say, "My ethics prevent me from doing this.", or even "I don't do this for religious reasons.", most people wouldn't have an issue with this. It's when someone says, "This behavior is clearly and obviously unethical/immoral.", that I see the issue, especially on things where there is gray area for a lot of people.
I think most of us can agree that murdering someone is unethical/immoral. But when we are talking about things like drinking, eating meat, dumping out baby food, it tends to get into a gray area. There are thing that I do in my life because of my personal moral code. But I don't necessarily agree that they are wrong or unethical in general.
For instance, I don't drink for religious reasons. And for me, that is important because of my personal beliefs. But I honestly don't give a crap if other people want to drink. I have no issue with that. So I really think that it's all in the tone. "Personally, I don't drink for religious reasons.", is much different than, "Drinking is immoral. "