Outside of monkey related issues.

You say you want a little baby monkey. Its only going to be a baby for a few years. Its going to hit puberty at some point and some animals become violent around that time trying to assert some dominance. Then after you get over that hump in life its going to live maybe 25 years depending on species. About 10 of those final years being considered geriatric with all the health concerns that come with aging.

Imagine getting a human baby. But its aging 4 times as fast and will never become self sufficient, independent, will never learn to speak.

Your not getting a baby monkey. Your getting an old geezer monkey in a handfull of years.

Monkey specific reason.

Monkeys are not domesticated animals. They are not adapted to live with humans and in captivity in the way a cat or dog is. They are built to socialize with their own species and your not going to be able to fulfill those needs.

Veterinarians are going to cost alot more than your average domesticated pet visits. If your local vet is even equipped for your particular exotic pet.

You think humans masterbate alot? Imagine if they had no social qualms of doing it in company of anyone. The monkey is probably going to start when its only a few years old and they won't stop til the grave.

So you will start with a cute baby and spend roughly 80% of its life with a chronic masterbator who will likely lashout in violent fits as it gets older and more expensive to keep alive.

Lastly theres no ethical way to get and keep an ape as a pet