I will pose a hypothetical - let's say you are a member of a tribe that practices homicidal cannibalism. You personally feel that killing humans for food is immoral. You are allowed under the customary law of the tribe to refrain from participating in cannibalism, but regardless of whether you do you will still have no power to prevent the tribe from killing and eating other humans.

Is your choice on whether to participate in cannibalism morally neutral? My personal feeling is that the answer is "no" - regardless of the fact that your actions ultimately have negligible impact either way, you have a moral obligation to abstain from eating other humans, because it's obviously wrong. Trying to justify it by saying "everyone else is doing it and I can't stop them" is absurd.

You're only convinced of your argument because you already don't have a problem with animals being killed for food - it's a post-hoc rationalisation. If you were a vegetarian or vegan you would feel upset by the prospect of eating an animal because you personally find it immoral, so you wouldn't do it. The fact that other people can't be persuaded not to do something you consider immoral doesn't justify you participating in the immoral conduct.

Your argument reveals a failure to understand how vegetarians and vegans feel about eating meat - they feel a sense of real moral disgust that you don't experience when you take bite out of a burger, etc. If you imagine something that morally disgusts you, e.g. cannibalism, then you'll see the flaw in your argument.