For so long Penny has always been the promiscuous women who has been always asking her out and she finally realizes she's tired of living that way. She's tired of being in control and wants to give that up to someone but she couldn't find the right guy. Well, that's when Sheldon steps in.
And for Sheldon after doing research for a couple of years he decided to try a few things out on Amy but he realized she wasn't into it. So, they broke up and Sheldon wanted someone who could take what he had to give. He has always watched Penny from afar and after being alone for a while he realizes Penny just might be that one he's looking for and so that's when he makes his move.
For the plot I was thinking that Penny either goes to Sheldon for help to fix her computer and while he's fixing it he finds in her internet history that he's been researching BDSM and makes a comment about how her research us poorly done on the subject.
OR one night he walks into her apartment because he maybe needs something and when he approaches her bedroom he sees that it's cracked, he walks in on her masturbating and offers his pointers on how to make it more enjoyable. Well, at first she gets angry and makes him leave but as he's leaving she decides to give in and try it and after she does she craves more of it because he is so skilled.
So then he decides to his advantage and that's when things began in the beginning. He knows how much she really wants it after the first time. She won't ever say it but he can see it in her body language.
And I would like to throw it the non sexual power play moments, like when she would do something right he would reward her with candy or something. For those who have seen the show know what episode I mean.
Sheldon't sex drive is definitely is very real and that's what shocked her the most in the beginning because she always believed he was completely against human contact. But, in reality he has always secretly wanted her but before she was dating Leonard.
I was thinking that maybe he would have a key to her apartment like she does to theirs and so it would be late at night when they have their encounters.