Backstory and how

Well Goku before the cell games got divorced to Chichi. Gohan knew but said nothing because deep down he was a bad boy. Gohan knew he wanted a harem and one that is immoral but not illegal. Gohan loved many girls he knew chichi bulma launch are to name a few. But cause goku he could never act on it. So Gohan got Vegeta to not marry bulma but go and train and hopefully train so much that he will surpass Goku the next time. So Gohan one day was given by Zeno in his sleep the Hentai system when at the same time giving goten his little brother the normal system. the Hentai system makes it for each girl he has sex with he gains 2 levels guaranteed.