Having sex with donkeys is a part of growing up for the boys in northern Colombia. There are numerous reasons given on why they do it. Some believe having sex with a donkey makes the penis grow bigger. Other people say having sex with an animal is the traditional way in many cultures, for a man to prepare for sex with a human woman.
This is not something new or even indigenous to Columbia bestiality and zoophilia are in every culture in civilization. Columbia is a very religious country so the girls don't start having sex until their late teens or early twenties. Some parents interviewed were happy the boys were having sex with donkeys instead of girls because they couldn't get the donkey pregnant.
The video below is an expose on the local Colombian animal sex traditions along with interviews with the locals and a Ph.D author on the subject. Here's a spoiler for you...it ends with a man having sex with a donkey.