Let me preface this by saying that I support spay/neutering, but your post has a lot of assumptions that aren't always true.
1- marking. We had our pug fixed at 4 months old. He is 5 years old now and has marked his entire life. Continually. Outside, he will mark something, then turn around and hit it from another angle. I've never counted, but I would guess that an average trip outside for 30 minutes will have him marking 50 times. Our GSD, who is 17 months old and is not fixed yet, didn't start marking anything till he was over a year old, and he doesn't mark with near the frequency the pug does. He might hit a couple objects while he's outside and has never marked anything indoors, where the pug will mark indoors if you don't watch him. The GSD didn't even lift his leg until he was 10 months old.
2- if a dog has aggressive tendencies and isn't trained on how to act around other dogs, fixing won't help. You can see examples of unfixed dogs who are not aggressive all the time in dog shows, agility, and other activities where dogs interact. Changing his personality is all subjective. A well adjusted, trained, loved dog should not have personality issues whether he's fixed or not. On the cat front, we just had our female spayed. She was a cuddly love bug before she was fixed. Now she's aloof and not nearly as lovey as she was before. She's still sweet but doesn't spend hours on your lap purring anymore.
3- There are studies that show that fixing can cause other health problems, so it's not a cure all for potential diseases later in life.
4- my friends had their cat fixed and she turned into the meanest, nastiest cat I ever saw. You can't walk into their house without the cat hissing at you and swatting you as you walk by.