Best example for this is when you can't have regular sex like you used to before you had kids. You could have pretty much done it anywhere anytime, but now that you have kids, it's a no go. They're always around. So you can only do it at certain times. Hence the term red light, green light.
red light green light sex:
Jack: Damn, never have kids man
Eric: Why not?
Jack: Because the sex is nothing but red light, green light!
Jack: I'm in the mood.
Lisa : Sorry honey, but can't. I have to go pick up the kids from school soon.
Jack: *fustrated* damn it Lisa, every since we had the kids, its been nothing but red light, green light sex!..