Why is there an opening?

From:  "Tessa"

Why do men & boys underwear have an opening in the front of them? This can not be used to pee through.

Nick was the first to come up with this theory: 

Actually the opening in men and boys underwear is used to pee though. It doesn't just magically pass through you have to move everything around a little but that is what they are there for. There's not really anything else that they could be used for.

KJ simply answered with this: 

The hole is for opening, then you pull out your ..., then use it for whatever purpose you like.

amr shalakani thought of this idea: 

good one! the idea is not about peeing through... the hole was made to allow for "distensibility"....guess why? lol

TB faithfully answered with this: 

During Victorian times, people were prudes and wanted ways to minimize exposing bare legs and skin. Flaps were provided in undergarments to facilitate this much like the long johns. Undergarments were originally a lot looser. With the introduction of briefs, manufacturers tightened up and shrunk the flap.

Catt responded with this: 

It is used to pee thru (tho you do have to unzip and pull "it" out first) its much easier to use a urinal, this way its easier to aim then having to take down and hold with both hands your trousers

Sp00k concluded in agreement with Nick: 

I'm sure there have been many reasons found for the opening in the front...but personal experience, and sheer convenience lead to believe that yes, it is used to urinate out of. Boys/men have extra parts down there...by extra I do mean external.