When they reach their teens or so, they'd have attained a considerable level of Independence, so your presence won't be needed like when they were younger.
And even if they don't know enough, they will take their independent path anyway :)
On the case of two parents, I think it is better to have both, but not always possible. But I also feel there is some difference if parents are lost through divorce, or through death like in a war. I think it has a different effect on the kid.