I think the first disappointment that new riggers experience is that "with automatic weights" is not infallible.
First, when you parent, watch the bottom center of the screen to see if you get the infamous "bone heat" message. But you do not have that problem.
Second, be prepared to learn about Weight Painting and Vertex Groups. You do have that problem.
When you select with automatic weights, Blender uses an algorithm to guess what part of your mesh should be controlled by each bone. But real meshes sometimes require that parts of the mesh be controlled partially by each of two different bones.
To solve this, Blender adds the parts of the mesh to vertex groups, one vertex group per deforming bone. But it can get that wrong and either use the wrong group or assign parts of the mesh to two bones. This is what's happening when you move your character's left leg.
Crash course in weight painting
- Select the armature
- Shift select the model
- Use the menu to go to weight paint mode.
- ctrl-click on a bone in the leg to select it.
- Use the subtract brush to remove anything but dark blue from anyplace except the leg you want the bone to control
The manual has better details and there are many tutorials.