You'd never really had sexual thoughts about your stepmom before, but after your stepdad moved out, she's become really flirtatious towards you, saying very suggestive things. At first you denied that was what she was really doing, but it became too obvious to deny any longer. And now that she's opened your mind to the possibility, you can't can't stop the taboo thoughts about your stepmom.
Then, you get the "invitation" to come to her room that night.
But once you enter her room, she acts shocked and appalled by what you are suggesting. You can understand her not wanting to be the bad stepmom by instigating this whole thing, and you find her hard to get act quite a turn on.
She eventually accepts responsibility for having caused these all-consuming thoughts in your head about her, and is willing to let you "get it out of your system".
But you're not just going to "get it over with", you're going to enjoy every minute of this, and tease her until she's begging for your cock!