Well, his dad had a really kinky giantess fetish. He's seen the snu-snu episode of Futurama and from that day forth, normal women couldn't satisfy him. He needed to know his partner could break his neck at any moment - it excited him.
He went along to knockturn ally and tracked down the infamous "Willy a Witch"" brothel.
He was directed to the back room, where Fridwulfa was lying un-sexily on a sofa. She had lost all her money betting at the dragon fighting ring, and had been forced into prostitution to cover her debts. After 40 minutes, 3 gold galleons and 2 black eyes... the deed was done.
He went his separate way, but she had failed to make use of her contraception (she'd sold it for more gambling money too) and found her self pregnant.
Anything in the actual books that contradicts this story is lying, m'kay?