It depends on the specific plot and character dynamics of the manhwa. Sometimes such a scene might occur if it fits the story and the relationship between the characters.
The plot usually involves a girl taking the place of someone as a bride, often leading to unexpected twists and turns. The main characters could include the replacement bride herself, the groom, and various family members or rivals.
It's a romantic tale about a girl who unexpectedly finds love with a prince. There are lots of sweet and challenging moments as their relationship develops.
It really depends on the context and the relationship between the characters. Sometimes it's a sweet and romantic gesture, but it could also be inappropriate depending on the situation.
A major theme could be the blurring of boundaries. Ahsoka is a Jedi, and Lux has his own path. Their relationship and this wedding - like moment in the fanfiction might explore how they break the traditional boundaries of their respective roles. It could also be about loyalty. They are loyal to each other and this loyalty is what brings them to this point of a wedding - like commitment.
A fanfiction about Lux and Ahsoka with the 'you may kiss the bride' scenario could be a really interesting take on their relationship. It might be a story where they end up in a situation like a wedding, perhaps in an alternate universe. It could explore their feelings for each other in a more romantic and committed context than what we usually see in the regular Star Wars lore.
Without having read the specific novel, it's hard to say exactly who the bride is. But she's likely a central character, and the fact that she wears black gives us some clues about her character, perhaps she's a non - conformist or has some sort of emotional baggage.