Source: http://www.justice.gc.ca/eng/dept-min/clp/faq.html "The Criminal Code provides "close in age" or "peer group" exceptions.
For example, a 14 or 15 year old can consent to sexual activity with a partner as long as the partner is less than five years older and there is no relationship of trust, authority or dependency or any other exploitation of the young person."
Edit to address your Edit: Because after having a lot of drinks at a party ('drunk'), and then having what could be considered strenuous physical activity (i.e. sex) would not induce vomiting if it wasn't rape, right? Look, I'm not at all condoning what the 4 male youth have done, but let's still use logic in our discussions.
Further Edit: Also I just want to add, unlike other Commonwealth Nations (e.g. Britain), the Canadian legal system does NOT allow extra charges to be tacked on while the legal proceedings are in motion. So before the RCMP and Crown Prosecutors initiate prosecutorial charges, they need to be relatively certain that they have enough evidence to win as they stand currently. They can't just say "Hey, I charge you with X with evidence Y, Z. Let's go to court." Then 5 days later say "and also A with evidence B, C."