You can safely lock your bike outside.
With knowledge and equipment

Your bicycle should normally live indoors. But it can be locked out on the street in a well observed safe area for moderate periods of time. You can be secure locking your bicycle outside while running errands, being out to a restaurant, or bar, or going to the theater or art galleries, with just a bit of forethought, and the right equipment.

The most practical device for locking your bike is a good mini u-lock. The best ones lock both ends of the u. Weaker u-locks (British call them d-locks) are broken by prying. Contrary to longer shackle u-locks, the mini u-locks are much harder to break by prying.

The hardened chains with the correct lock (often called a disc lock) are very secure, but are cumbersome to carry. Even the biggest hardware store chain and biggest padlock can be cut with ordinary theft tools. Only noisy power tools can cut the big hardened chains and mini u-locks.

Also, using two locks more than doubles your security.

Very Secure Bicycle Locking Method
excellent for iride bicycles

Here is an example of a rather secure bicycle. That type of lock is very resistant to most attacks. The wheels are very well secured to the frame with locking skewers. The valuable carbon fiber fork is locked in place. And the immovable object cannot be breached.

Secured front urban bicycle wheel with locking skewer
open only with key

To make locking up much easier, secure your wheels with locking axle skewers or special nuts for bolt-on axles.

Without these, wheel theft will often happen with no tools. These skewers require a uniquely shaped key to operate. And it shows; the retainability of these wheels is obvious to every person walking by.

Secured rear urban bicycle wheel with locking skewer
open only with key

These "locking" axle skewers really work. There are a few brands of security skewers available. They replace the quick release skewers.

With the wheels secured to the frame, locking up your bicycle is a lot less stressful. You don't need to worry about the wheels, just look toward locking the frame securely. Easy.

Close up detail of the well secured bicycle.
Connecting the pieces

In this detail view we can see how this bicycle is so well secured. The lock cannot be slipped over the immovable object, and there is very little space for prying tools.

This is super well secured because the valuable carbon fiber fork is also locked with the frame. The fork can be secured if the front wheel has a locking skewer or nut, and a lock is connecting the frame and wheel. The fork cannot be removed from the wheel, the wheel cannot be removed from the frame.