This reveals a challenge to socially instituted standards.
Why do brides get married in a white dress? Because at some point in the past, this was institutionalized as something good, which represents the purity of the bride. Notice that there is no law that obliges the bride to marry in a white dress, this is just a socio-cultural issue.
The point is that not all people are comfortable in accepting socially instituted standards. The bride can choose the dress she wants, including the colors of the rainbow.
The same applies to the fact that a female friend is the best man and not the maid of the marriage. In this case, it is the question of gender. There is a social construction of which people of the biological sex masculine are men and the people of feminine biological sex are women. This applies in general, but there are exceptions. Science has already proven that some people are transsexuals, that is, they have biological sex different from gender identity.
In short, what this couple seeks is to disassociate their marriage relationship from what is socially considered normal. And the marriage is theirs, so they must do what they feel like. What matters is that they are happy!
