Years ago, you promised your firstborn to a witch. Since then, despite your best efforts, you can’t seem to get laid. The witch is starting to get pretty pissed.
Y’all get together to discuss your options and she starts coaching you on how to get men because she doesn’t want to waste more magic on you without promise of payment. The more time you guys spend together the more you realize you have a bit of a crush on her. Soon you’re sabotaging your dates on purpose to see her again.
Long story short you fall in love and get married and do the sperm donor thing AND YOUR FIRSTBORN IS HERS BY DEFAULT and you live happily ever after. The end.