Your position seems to be that we should adhere to the superstitious demands, regardless of how onerous, made by society, as long as it's big enough.

So, for instance, if the majority in the U.S. decided that black people shouldn't get human rights because they're "not fully human", that's totally fine--who are we to say they are, right? (Note: this was an actual issue during slavery and, regionally, for decades thereafter.)

Or witch-hunts. Who's to say the women aren't witches? Totally appropriate behavior.

The tyranny of the majority is a terrible problem. It's one of the primary reasons that the idea of "human rights" was invented! Human rights aren't a natural thing. They're a societal thing, an agreement that we treat people at least with a certain minimum standard.

The arguments for human rights are varied, but the ones that don't require more than the most minimal commonality of moral intuition are based around the idea being an individual. For instance, a common (and reasonable) thought experiment is a permutation experiment: if which individual you were was permuted randomly, would you still be satisfied that your society was adequately fair?

But being an individual is a phenomenon. It's not a magical label we paint with no measurable difference, like, say, labeling half your forks male and the other half female. There are a whole bunch of capabilities that go along with being an individual: reaction (at least internally) to stimuli, experiencing those stimuli, and so on. Science has shown that these things are implemented by the brain. And it also tells us when the architecture of the brain could possibly or almost surely cannot support such things.

So you have to go back and actually look at the evidence.

Arguing otherwise is--as you are doing--arguing for nothing less than the vacuousness of all human rights, in favor of, apparently, whatever belief people end up with for whatever reason.

Now it is true that what is good enough to count as an individual is somewhat arbitrary. There isn't any particularly interesting distinction in individuality between a 15 week old fetus and, say, some chunk of human tissue. But there are substantial differences between a 24 week old, a 6 month old, and a 3 year old. In principle, we could choose a later point if we were sufficiently confident in our decision and we could avoid other extremely detrimental consequences based on differential treatment (neither of which is true right now, incidentally, but you could imagine it).

The 24 week thing--or something close to it--is the most conservative one can be with the individuality arguments before it completely falls apart. The brain structures necessary for integrating experience simply aren't there until about that time.

Anything earlier developmental stage than that is no better justified than would be a law that you have to keep kosher: no preparing meat and dairy in the same place, for example.

Or, if it is better justified, fine: present the justification. But that's the key. It has to be justification. Not "I feel like demanding this of other people". Not "I feel that squirrels are people too". Give an actual justification based on something.