Well, if I was uncomfortable talking about sex I'd probably put 'Something About Mary' on, and when the masturbation scene came up laugh and say "it's incredible how those idiot JW's can say something as natural and healthy as masturbation is wrong" and improvise from there.
If I were a teenager being approached in the rather formal way you suggest would freak me out a bit. Maybe that's me...
My daughter (now seventeen) came to live with me last year and has just about got used to my fiance fervent 'been masturbating since, well, seven or something' jill-off advocacy. She's thus far avoided being dragged into a vibrator showroom by her; having received one from her mum at 14 she's keen to do the same to her step-daughter. We just talk about sex very openly and my daughter now reacts and responds like a young adult instead of the rather more schoolgirlish attitude of before (icky! or giggles)
I think it is just possible they may know masturbation is perfectly normal and are quite good at it but will be dreadfully embaressed their talking to a parent about sex.
That only disappears if you do it so it's not a big deal anymore; whether that exposure is through humourous, direct, or formal methods.
Oh, I'd also recommend leaving a big-ass sex manual lying around; something like the 'The Guide to Getting It On' by Goofyfoot Press.
How many of you actually had good communication with your parents about sex?
Not me
What worked?
Nothing
What was stupid?
Everything
What did you wish they had told you?
Something useful or untwisted by coyness or religious prudery.
What should they have left out?
The Chapter in the Great Teacher book that represented my sex education outside school