The annoyance is fascinating to me, because I grew up eating meat and also felt such annoyance. It's an interesting thread to pull if you're into knowing thyself - why do I feel especially aggravated on this topic? Might lead you somewhere.
I don't find this equivalence very compelling. Anti-gay arguments are based on either religious or extreme-naturalism grounds. This seems much different than arguments based around not harming sentient creatures, which I think many people intuitively identify with to some degree.
You argue philosophy (religion is a philosophy like any other in my book) but an existentialist would tell you that the morality of eating meat is purely an artificial construct: the animal simply exists and the actions of killing the animal and eating it are just actions.
Or, to put it more bluntly: you can't prove that eating animals (or gay sex, for that matter) are the "wrong" choices to make, no matter how much philosophy you've read or argued about. You can't even prove you exist or that this is reality. If you can't even prove that, you're a long way from proving that killing animals is a moral choice.
I think you misunderstand the nature of argument. All arguments rest on a set of premises, and those premises themselves rest on premises, recursively so until we reach axioms which we simply accept to be true without further justification - like that the external world exists in some sense. Feel free to doubt that, by all means. Proof is not accessible to us - not a point the existentialists made (as you seem to believe), but rather something they took as self-evident and were responding to.
Analytic philosophy basically contends that if we assume axioms hold (correspond to the world in some sense) and we construct arguments from those premises which conform to logical rules, then the conclusions of those arguments are also true of the world in some sense. This is, of course, doubtable in various ways - but you pretty much have to abandon reason itself as the alternative.
Our current scientific understanding in no way leads to such a conclusion.
However, even considering the hypothetical that plants are sentient, eating plants actually causes the least harm seeing as livestock are not very efficient converters of plants to meat.
>eating plants actually causes the least harm seeing as livestock are not very efficient converters of plants to meat.
Let's continue this facile logic to it's conclusion: if animals are causing so much "harm", why shouldn't we kill them to prevent them from "harming" so much?