I feed my cats and bunny food that is made for them. There is so much, I have the Oxbow hay, science selective food, spring greens, herbs, cucumber etc.
The cats have chicken, prawns etc and Jacob has the odd grape, slice of banana apple etc though less fruit and more veg. I spend hours searching for the best stuff for them food, bedding, toys etc. My boys mean the world to me and I think part of loving someone is to accept them for who they are. My boys aren't human so they don't have human food (though they have human quality). Jacob loves to run off with my hot cross bun if the phone rings and I leave it unattended but I don't 'feed' it to him. With porridge I was under the impression that it is a good short term food for ill or elderly bunnies who needed to build their strength up, it isn't an everyday food.
I have a croissant for breakfast so no cereals anyway but it wouldn't have occured to me to feed Jacob it as it is human food. It's not just weight gain, teeth suffer with high sugar and frankly bunnies/cats/dogs are biologically made to consume sugar so the long term damage of their bodies trying to process an alien substance doesn't bear thinking about.
I used to get Jacob those sugary bunny treats as marketed for rabbits. I know better now, rabbits don't need 'sweets' and the weight is hard to get off. I can say hand on heart that Jacob is as happy with a bit of basil and parsley as he was with sugary treats and I feel better knowing I am treating him as a rabbit and not as a human friend. There are so many nice bunny things, the oxbow hays, veg, herbs, dandelion leaves, readigrass etc that are good for bunny in so many ways, why spend all that time trying to see if coco pops or whatever are OK??