I guess if one were to debate this seriously, the only logical conclusion would be something to the effect of: “Of course you should maintain a healthy diet for your pets. Insuring a long healthy life is paramount. Your personal eating habits are irrelevant”.
I don’t disagree with that, still though. Seems a little hypocritical. 
Fair?
What does “fair” have to do with taking the best care you can of the creatures entrusted to you?
Nothing, as I pointed out in my OP.
If “fair” has nothing to do with it, then how do you expect people to answer the question ”Is it fair?”
It was a funny conversation I was having with my sister this morning. I wasn’t really expecting anyone to take this conversation seriously.
The point I was making with my sister was. You wouldn’t wolf down a meat lovers pizza in front of your kids while feeding them a plate of vegetables. So why is it different for your pets?
Replace “pets” with “children”. And “diet” with “tobacco smoking habits”. Is the answer obvious now?
Pets don’t learn by example the way that kids do. But that’s not what the OP was asking.
Until he learns how to crap in the toilet like the rest of us he’ll eat what I give him.
Are you asking why specially-formulated pet food is healthier than greasy fat, sugar and carb-laden human food?
Apart from some pets being pickier than others, pets in general don’t see the difference in food that we do. I think Cecil actually covered this at one time, but humans have far, FAR more taste bud than our pets could ever hope to have. My cat Skittles wolfs down his dry kibble like it’s a 12oz ribeye at the most expensive joint in town. If anything I’m actually pretty jealous of him, lol. I never get that excited about the shit I make…
Ah, here it is.
My dog is in MUCH better shape than either of his human parents, because we find it much easier to discipline him than ourselves. Like the OP, I have often noted the irony of the situation. Sometimes I wish that I had an overlord that would take care of my health as well as I take care of my puppy’s.
It’s just another example of where they are lucky, like not thinking of their mortality in the abstract. I think it unlikely they sit around thinking of particular tasty but bad-for-them stuff they’d like to eat. If it’s not offered, they probably don’t think about it like we would.
Also depending on the dog (our main experience) some of them you have to encourage to eat enough to maintain weight. My daughter’s dog is like that, gets healthy food and supplements, really beautiful slim, muscular, shiny, frisky ‘pit bull’, but you have to put treats in her food for her to eat it all. There’s no tendency at all for her to overeat unless you fed her stuff she shouldn’t have. Other dogs we’ve had would eat as much of their regular food as you’d give them, till they exploded, seemed like.
Pets can’t fend for themselves. It’s fine for you to kill yourself with poor eating habits, but killing an animal through obesity is just cruelty.
Is it fair to force your pets to stay on a healthy diet when you, yourself can’t stay on one?
No, but fortunately pets are unable to lodge a formal complaint about it. Just don’t rub it in their faces.
If you’re eating an unhealthy diet or have other unhealthy habits, you’re harming yourself. I can’t see any moral argument that says you should balance out the harm you’re doing to yourself by also harming those around you.