Humans don't like to eat other humans cause they don't want to be eaten themselves or know that people close to them are going to get eaten. Plus, there is no... cultural... background for people to think its a good idea.
The cannibal cultures all probably died out over time.... disease and/or haters hating on their lifestyle.
Eating things that can understand you and talk back? Now, that is something that will go either way when we get there. ALOT of people(Developed World more likely) will say, "DON"T EAT EM!" and others(Starving Peoples of Poor places more likely) will say, "THEY IS GOOD FOOD! I HUNGRY!"
We know who is going to get the final say. The people with the bigger guns.
Animals though, tend not to transfer disease/poison when eaten, which makes em eatable, unless they are poisonous or fatal when ingested. Plus, some of em are tasty and most of em are filling to the stomach, staving away hunger.
And there is the half-arsed justification which billions of humans still use. I use it too. I don't need any more reason. I do not feel any guilt.
Pets: People don't like other people trying to eat their companions. It won't catch on.