You're off to college soon, 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 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.