You're off to college soon, Step-mommy will be all alone in the house, she's going to miss you so much it makes her sad. She has thought of a way that she will be less lonely when you're gone, if she got pregnant, then she would have another Step-son to look after, to nurse, feed, dress... You say you are fully supportive of the idea, a sperm bank will be a great option for her, but she doesn't trust a sperm bank, there is only one person in this world she would trust to breed with, one persons who's cock she made, she created, birthed, nurtured and that's you.