Remember if you want some option in toogle groupe to be always active, then specify what toggle shoud be active if all of them go off.
public bool turnOn;
void OnEnable()
{
GetComponent<UIToggle>().optionCanBeNone = true; // <- This is your problem!
turnOn = GetComponent<UIToggle>().startsActive;
}
void Update()
{
GetComponent<UIToggle>().value = turnOn;
}
Also remember to sendout status change on UIToggle value change event.
But as i see it, its a lot more easy to use toggle with out any other scripts and sendout c# event on value changed or use NGUI EventDelegate system.