So I think premise 2 is probably false. If we did some research and it turned out that human penises were designed to rape, that shouldn't make us think that rape is sometimes ok. (Actually, iirc there is some theory that the penis head is designed to scoop out other's sperm, so who knows).

But either way we can grant the argument and still accept ethical veganism, since we probably aren't designed to farm animals on factory farms, and veganism is generally only concerned with ordinary circumstances.