Differentiate between "all moral positions are non-rational" and "all moral positions at root are non-rational" (I've emphasised the significant bit for you).
I did indeed claim that that drawing the line at plants vs animals was more rational than drawing it at any other point, because it is indeed more rational to draw the line there for the purposes of ethical vegetarianism, what with ethical vegetarianism being concerned with minimising the suffering caused to others, that by implication those others need to have the capacity for suffering, and therefore it's perfectly logical to draw the line at this capacity. It would be illogical not to draw the line there, as it would be to draw it elsewhere.
Of course, as the Anti-Chris and kenminot ( x2) have explained, this position results from a non-rational concern for the suffering of others, which appears to be more-or-less hardwired into us. That this concern is non-rational doesn't make the position which satisfies it non-rational, or illogical; in fact, it would be irrational and illogical to address this concern by other means, eg. stamp-collecting or slaughtering livestock.