God was tired of seeing the world he created, continue its descent into chaos. He had to do something. It was clear that the devil Lucifer has essentially won.
God figured the only way he can win people back was to be the opposing force instead of being the one in power. So He decided to make a bet with his fallen son Lucifer. The bet was if God as the "bad guy" manage to convince the majority of people to follow the ten commandments Lucifer would stop interfering with his plans, if Lucifer manages to prevent the spread of God's commandments then God would surrender to Lucifer and give Lucifer what he had wanted when he rebelled.
God re-created the world and allowed Lucifer to select 2 people to become the new Adam and Eve and re-create humanity as he wishes.
Lucifer selected 2 NEETs, Gabriel a skinny horny introvert man who Lucifer plans on turning into the new Eve and Emily a average horny introvert woman who will become the new Adam.
For Lucifer he knew the easiest Sin to follow was lust, but he wasn't satisfied with just having a horny man and woman have sex, he wanted to turn it into something they try to fight against but get addicted to. One way he knew would work was have them slowly turn into the opposite sex. The change their bodies past the average human constraints whether that meant giving them oversexualized human bodies or turn them into monsters.
God had allowed Lucifer to create his "Garden of Eden" and thus can do what he wishes with it.
Lucifer introduced two rules in the Garden that any creature that Gabriel and Emily interact with will infect them causing an sexual evolution to happen to them and every time they have sex their bodies will start the slow transition to the opposite gender and slowly become hyper sexualized.
Lucifer asked one last thing for the bet, Lucifer asked that Gabriel and Emily become immortal and invincible, God acquiesced to Lucifer, making Gabriel and Emily immortal and invincible.
God transferred Gabriel and Emily to the new "Garden" and allowed Lucifer to take it from there. Meanwhile God planned how he will tempt one of them into "sinning" against Lucifer.