Placing suffering as the absolute centerpiece of morality and then basing all your conclusions on that.
Or an argument to "common sense" in that most people will tell you causing suffering is bad. Therefore it's bad.
I don't see either of these as being great arguments.
> In fact speciesism is illogical
Why? Most of it is perfectly logical. Pigs aren't humans. It's not illogical to treat them differently from humans in very substantial ways.
Also basically nobody says "we can kill pigs because I don't feel bad when I kill them". It's almost always "I do it for the meat".
And if you watch how we treat animals we largely differentiate them based on their functional differences. Even where you get weird cultural things like India not wanting to eat meat - that originated from the need for milk, fertilizer, etc.
The only weirdness you'll see is if you challenge people on it from an empathetic standpoint. People are socially wired to say there "good people who never would made an animal suffer" so you're running into social norms more than anything there.