As described here http://docs.oracle.com/javafx/2/ui_controls/toggle-button.htm ToggleButton and RadioButton do not behave the same way when in ToggleGroup.
However there are time when we do not want to be able to have a ToggleGroup where there is no unselected ToggleButton (see https://forums.oracle.com/forums/thread.jspa?threadID=2288865 and https://forums.oracle.com/forums/thread.jspa?messageID=10383377)

Currently we have to rely on hacks such as consuming the MouseEvent.MOUSE_RELEASED event on the button to prevent unselection gesture from being propagated to the button itself.
It would be good if we could specify a policy on the ToggleGroup or the ToggleButton to prevent this from happening instead.