A lot of the fanfictions about their kiss also explore the reactions of the other characters in the Voltron team. Some might be supportive, while others could be initially shocked or confused. The kiss can also be a symbol of a new era in the team's dynamics, as Keith and Lance's relationship changes. It can lead to interesting storylines about how the team adapts to this new development.
One common theme is the overcoming of internal barriers. Both Keith and Lance have their own insecurities and issues that they need to deal with before getting to the point of a kiss. Another theme is the exploration of their different personalities coming together. Keith's more brooding nature and Lance's more outgoing personality can create interesting dynamics in the lead - up to the kiss.
The popularity of Keith and Lance kiss fanfiction might be due to the strong chemistry between the two characters in the original source. Their personalities are quite different yet complementary, which makes the idea of them getting closer very appealing.
You can try Archive of Our Own (AO3). It has a vast collection of fanfictions including ones about Lance and Keith. Just search for the relevant tags and you should be able to find many stories with their kiss scenes.
One popular one could be 'The Unexpected Bond'. It beautifully portrays the relationship between Lance and Keith leading up to their kiss. The story builds on their shared adventures and how they gradually realize their feelings for each other.
One popular one might be 'A Moment of Truth' where their kiss is a culmination of a long - built - up tension during a battle - scarred journey. It beautifully captures their emotions.
They might have all sorts of adventures. Maybe they go on a family fishing trip and Keith has to learn how to bait a hook, which he's never done before. And Lance's family could be showing Keith all the local hidden gems in their town.