Just saying "ethical reasons" has never worked for me. It always leads immediately to "like what?" and devolves into a really annoying back and forth that I was trying to avoid in the first place. I usually just say "I don't like the way the animals are treated so I choose to eat other things instead."
Yeah, I don't think I've ever actually said "ethical reasons" specifically, unless someone poses the question "why are you vegan? For health or ethical reasons?" If it's an open-ended question, like just "why are you vegan?", I usually just shrug and say something like "I don't like eating animals," and leave it at that. Maybe there will be someone there who will ask why and I'll usually let someone else answer the question, as someone else at the table inevitably does because such questions seems to fire up conversation for everyone else. I don't actually ever feel a need to discuss it or get firey or passionate about it since my own mind on the topic is made up *dusts hands* and my discussing it in a debatish way seems futile to me. I usually just keep quiet and let other people throw the ball around when such talk starts, even though my prescence at the table is the thing that sets it off.
That said, I went for dinner Saturday night at my (omni) friend's house and they made a whole Vegan Thanksgiving dinner for me and all their (omni) guests: Tofurky, gravy and all sorts of sides and a cashew pumpkin cheesecake for dessert. It was amazing! Everyone seemed to enjoy all the food, too, not just me!