The only way I can see to open the selected bookmark in a new tab is with a keystroke (control-click, or control-shift-click). I'd like to set Safari to automatically open a selected bookmark in a new tab. Other browsers seem to have this capability.
If you have a mouse with a scroll wheel, you can push the scroll-wheel like a mouse button. This is a middle-click, and it will open a link in a new tab.
If you have at least a two-button mouse, you can right-click and choose to open a link in a new tab from the context menu.