Your step-mom is DESPERATE for you to think that she is cool. She's always tried to be more of a friend to you than a "step-mom," which is why it hurt her to realize that you have started masturbating and didn't tell her about it. Cool step-Mom wants you to feel like you don't have to hide anything from her, including that. She encourages you to take your cock out and stroke off in front of her, which gets her so turned on that she takes her clothes off and joins you for a fun mutual masturbation session!