One way is that it's a very hesitant kiss. Goku, being naive about love, is unsure but Chi - Chi initiates it, and it's described as a soft, brief touch of lips. Another description could be that it's a spontaneous kiss. They are in the middle of an argument about Goku's training, and suddenly, in the heat of the moment, Goku kisses Chi - Chi to stop her from being angry. It's a bit clumsy but full of raw emotion.
In a lot of fanfictions, the first kiss is a discovery. Goku has always seen Chi - Chi as a friend, but during a festival in their village, he sees her in a new light. He watches her dance and laugh, and something clicks inside him. He walks up to her and kisses her for the first time. It's a kiss that surprises both of them, but it also starts to change the nature of their relationship. This discovery - type kiss is often written with a lot of detail about Goku's inner thoughts and Chi - Chi's blushing reaction.
In some fanfictions, Goku and Chi - Chi's first kiss might happen after a big battle. Goku realizes his deep feelings for Chi - Chi. He looks at her, and there's a moment of silence. Then, without a word, he just leans in and kisses her gently. It's a simple yet very emotional moment.
Maybe in this fanfiction, Goku realizes that he and Chi - Chi have very different personalities. Chi - Chi is often more concerned with family and a stable life, while Goku is a free - spirited adventurer. This clash of values could lead him to decide not to marry her. For example, Chi - Chi may want Goku to stay at home and help raise Gohan, but Goku is eager to explore new planets and fight powerful enemies.
His life might be more focused on his training and adventures. Without the family responsibilities of being a husband and father, he could freely travel the universe seeking stronger opponents and new challenges. He might also be more involved in protecting the entire universe rather than just his family.
One reason could be his pursuit of stronger opponents. Goku is always eager to test his strength and find new challenges. In fanfiction, this might lead him to leave Chi - Chi to travel the universe in search of the toughest foes. He might feel that staying at home with his family restricts his growth as a fighter.
I'm not sure if there is an actual fanfic with that exact plot. Fanfic topics can vary widely, but this seems like a rather unfaithful and out - of - character concept considering Chi Chi's strong bond with Goku in the original series.
Love and family are very common themes. Their relationship as husband and wife, and as parents to Gohan and other children is often explored.
Family is a common theme. Since they have a family in the Dragon Ball series, fanfictions often explore how they raise their kids. Another theme is love, of course, showing the depth of their relationship.
Well, in fanfiction, Goku kissing Chi - Chi can be a way to explore different scenarios. For example, it could be a moment of reunion after Goku has been away for a long time on a tough battle or training. This kind of scenario can bring out a lot of emotions, both from the characters themselves and from the readers who are fans of their relationship.
Well, their meeting in fanfiction could be really sweet. Maybe Goku finally realizes how much Chi - Chi has been there for him all these years. He might see her in a new light, beyond just being his wife. They could have a deep conversation about their past adventures and how they've grown as individuals.
A third possible plot is that Goku and Chi - Chi meet in a parallel universe where their relationship is completely different. They could explore this new dynamic and learn from each other. Maybe in this universe, Chi - Chi is a powerful warrior herself, and they team up to fight a common enemy, which changes their relationship in their own universe when they return.