system
Given the following post, respond as if you're commenting on Reddit:
human
Are there any arguments arguments against the right of animals to life that don't hinge on moral agency?

Hey, so excuse me if this isn't the right place to post this, but I couldn't exactly figure out where else I would. I've been vegetarian for over a year now and never really thought about it from a moral standpoint - it waa mostly a challenge for myself. But recently I've started thinking about it and looking into moral debates around the topic. I haven't looked into it as much as I'd like, but I am honestly deeply disturbed by some of the arguments I've found. While I've been vegetarian for some time I've never thought about it from any kind of ethical standpoint, and I do think a lot of the arguments in favour of it are flawed, but a main argument I've seem popping up in a lot of criticism's of vegetarianism/veganism is the idea that animals lack the same right to life as humans (something I'd previously taken for granted) because they lack the moral agency of humans. This deeply disturbs me, because it seems to suggest that some human beings (e.g. young children, people who have certain mental conditions or are cognitively disabled) lack the same right to life, or are in some way "not people". While admittedly I have to look more into this, the only arguments I have seen against it so far have been based on the idea that these people are ascribed value by the people who do have moral agency, and thus have no inherent value. As someone who has always considered myself outspoken in support of children's rights and the rights of all disabled  people, I find this suggestion to be kind lf disgusting. 
Putting that aside, I intend to look more into this topic once I've asked here, but I figured this would be a good place to at least pose the question, as this seems to be the primary discussion point around which a lot of debate around vegetarianism hinges. Are there any arguments against animals having the same right to life as humans that don't rely primarily on that right being directly linked to moral agency (the individual's ability to make moral judgements and be held accountable for them)? Forgive me if I'm misunderstanding the idea of moral agency, I'm not very experienced on this topic.
gpt
The argument you used is the "argument from marginal cases". Here is Peter Singers version of it. 

1- In order to conclude that all and only human beings deserve a full and equal moral status (and therefore that no animals deserve a full and equal moral status), there must be some property P that all and only human beings have that can ground such a claim.

2- Any P that only human beings have is a property that (some) human beings lack (e.g., the marginal cases).

3- Any P that all human beings have is a property that (most) animals have as well.

4- Therefore, there is no way to defend the claim that all and only human beings deserve a full and equal moral status.

Singer does not defend his first premise, but does not need to; the proponents of the view that all and only humans deserve a full and equal moral status rely on it themselves. In support of the second premise, Singer asks us to consider exactly what properties only humans have that can ground such a strong moral status. Certain properties, such as being human, having human DNA, or walking upright do not seem to be the kind of properties that can ground this kind of status. For example, if we were to encounter alien life forms that did not have human DNA, but lived lives much like our own, we would not be justified in according these beings a weaker moral status simply because they were not human.

However, there are some properties which only human beings have which have seemed to many to be able to ground a full and equal moral status; for example, being rational, autonomous, or able to act morally have all been used to justify giving a stronger status to human beings than we do to animals. The problem with such a suggestion is that not all human beings have these properties (humans with severe cognitive disabilities, infants, the senile are less rational than many animals). So if this is what grounds a full and equal moral status, it follows that not all human beings are equal after all.

If we try to ensure that we choose a property that all human beings do have that will be sufficient to ground a full and equal moral status, we seemed to be pushed towards choosing something such as being sentient, or being capable of experiencing pleasure and pain. Since the marginal cases have this property, they would be granted a full and equal moral status on this suggestion. However, if we choose a property of this kind, animals will likewise have a full and equal moral status since they too are sentient.

The attempt to grant all and only human beings a full and equal moral status does not work according to Singer. We must either conclude that not all human beings are equal, or we must conclude that not only human beings are equal. Singer suggests that the first option is too counter-intuitive to be acceptable; so we are forced to conclude that all animals are equal, human or otherwise.