Are people who are vegan, and also make their pets vegan( by giving them vegan anima food) doing something ethically correct since their pets can't really decide if they want to be vegan or not, and it's against their natural behavior to be vegan?

Pets already can't decide whether whether they want to eat kibble, whether they want to be walked on a leash, whether they want to stay indoors most of the day, whether they want to hold their water in until their owners get home, whether they want to be spayed or neutered, nor, for that matter, whether they want to be pets at all.

Pets are different in a lot of ways from their undomesticated counterparts. If interfering with their "natural behavior" is wrong, then keeping pets at all is already a problem. So long as the vegan pet food is nutritionally adequate and doesn't cause the animal any distress, it's hard to see why it would be more wrong (if wrong at all) than any of the other things we've mentioned.

Read another response by Allen Stairs
Read another response about Animals, Ethics