I would not recommend seeking out such fanfiction as it goes against the general spirit of the show, which is about love, redemption, and the battle between good and evil. The relationship between Chloe and Lucifer is much deeper and more nuanced than just a physical encounter, and most fan - created works aim to explore that depth rather than something so vulgar.
Well, their love story begins with Lucifer being drawn to Chloe in a way he couldn't quite understand at first. Chloe, on the other hand, saw Lucifer as an enigma. They worked on cases side by side. With each interaction, their feelings for each other became more intense. There were times when Lucifer's devilish nature put a strain on their relationship, but Chloe's acceptance of him, flaws and all, was a turning point. Eventually, they fell deeply in love.
The theme of Lucifer's struggle with his devilish nature and Chloe's influence on him is also quite prevalent. Chloe represents the good and the human side, and she often challenges Lucifer to be better. Fanfiction might explore how he changes over time because of her. Also, the idea of their eternal bond, whether it's fated or just a strong connection they've developed, is a popular theme.
Another theme is the battle between good and evil. Lucifer, being the devil, constantly struggles with his nature. In fanfic, this can be shown in different ways, like how Chloe influences him to lean more towards good. It also delves into the idea of redemption, which is related to the good - vs - evil theme.
One key moment was when Chloe saw Lucifer's vulnerable side for the first time. It made her realize there was more to him than meets the eye. Another was when Lucifer chose to protect Chloe at great risk to himself, showing the depth of his feelings. Also, when they finally admitted their love to each other was a crucial moment.
One potential antagonist could be Lucifer's father. He might not approve of the union or the baby, seeing it as a disruption to his grand plans or as something that could change Lucifer in a way he doesn't want. Another could be some of the demons from Hell who see the baby as a potential threat to their power or existence if it has some sort of unique abilities.
Another scenario could be Chloe getting hurt by Lucifer's past actions coming to light. For example, if a victim from Lucifer's wilder days shows up and Chloe realizes the extent of his former recklessness. She might feel betrayed and deceived, leading to a rift between them. It could be a complex situation where Lucifer has to work hard to regain her trust.