Relationships between step-parents and stepchildren are never easy.
When someone marries a person with children, they not only become a new spouse, but also a new step-parent. And depending on whether the children have the possibility to see their birth parent or not, they will be closer to the new step-parent or simply cohabit the same space. Some other times it doesn't even depend on that. The age of the children, the relationship they have with their birth parents, and many other factors can contribute to how close step-parents can become with their new children.
This new living arrangement can be more difficult for some, and there's not a single way to handle friction between new members of the family. There's not a set of feelings that is exactly like others either. Some members might feel more connected to their new family, and the rest might feel differently. There's nothing inherently wrong with it. Everybody has a different way of feeling and experiencing relationships, and in this case, the stepdaughter and stepfather were feeling very differently towards each other.
She lost her father when she was 12 years old, and her mother married another man by the time she was 15. Even though this man never adopted her, he took over and even paid for some of her schooling. They get along just fine, but she never thought of him as her father. Now that she's getting married, she asked her uncle, her father's brother, to walk her down the aisle. Suffice it to say that her stepfather was very disappointed. They obviously feel very differently toward each other.
We can understand why the stepfather was upset, since he's been in her life for quite some time now. But at the same time, no one can ask her to do something she doesn't feel on her own wedding day. They are both entitled to their own feelings, and both sides are valid. The goal here would be for them to have an honest talk and find a solution that doesn't hurt anybody.