Best Woman

In a normal movie wedding you have the bride, the groom, the best man, and the maid of honor, along with various groomsmen and bridesmaids. But what if the groom's best friend is a girl? Or the bride is a tomboy who hangs out with all guys?

This trope describes the events and confusion when you have a role that traditionally (and only traditionally) is one sex, being filled by the other.

Examples of Best Woman include:

Film -- Live Action

Made Of Honor [the Patrick Dempsey movie] is about a male bridesmaid. It's mentioned in the film, too, that if he had gotten married first, the bride would have expected to be his best man.
Similarly, in My Best Friend's Wedding, the groom's father mentions to Julianne that he had suggested that groom make her his best man, but the bride-to-be wanted her for her maid of honor.
Happens in Seltzer and Friedberg's Date Movie, where Grant's best man, Andy, turns out to be his sexy ex-girlfriend as well. Julia finds this out the hard way.

Live Action TV

In Gilmore Girls, Rory was the best man at her grandparents' vow renewals.
In Dexter, Debra is Dexter's best "man". She's his foster sister.
In Ugly Betty, Bradford passes up his son Daniel as best man and instead asks his transsexual daughter Alexis to stand up with him.
On Buffy the Vampire Slayer, Xander picks Willow as his best "man". Of course, her being a woman does not stop her from flirting with one of the bridesmaids. She does try to use it as an excuse to get out of her horrible bridesmaid dress, though.

Willow: I'm supposed to be best man. Shouldn't I be all ... Marlene Dietrich-y in a dashing tuxedo number?
Buffy: No.
Willow: Oh. [pouts]
Buffy: That would be totally unfair. We must share equally in the cosmic joke that is bridesmaids-dom.

On Night Court, Bull has Roz, his best friend, play this role at his wedding.
On General Hospital, Brenda served as both the best "person" and maid of honor at Ned and Lois' wedding, being the best friend to both of them.
On NCIS, though the wedding hasn't happened yet, the person Palmer has tapped to be the best man at his wedding was Abby.

Music Videos

In the video for Katy Perry's "Hot 'n' Cold", one of the bridesmaids is a grubby, stubble-faced man.

Web Comics

The tomboyish lesbian friend of the male lead of Marry Me is his best "man" in the epilogue.
In Girls with Slingshots, most of the groom's friends are women, so during the wedding, we have a train of "groomsmaids".

Web Original

The "male bridesmaid" version of this has 214 story examples (and counting!) at FictionMania. Invariably, the male lead is required to wear clothing appropriate for the bridesmaid's role.

Western Animation

In the season 2 finale of My Little Pony Friendship Is Magic, Twilight Sparkle's brother Shining Armor asks her to be best mare at his wedding.

Video Games

In The Sims Medieval, the "Wedding" quest has two roles for Hero Sims, the Groom or Bride and a Best Man/Maid of Honor. (You choose the Sim the groom/bride marries, but don't control them.) The two roles can be played by two Sims of the same sex, or of opposite sexes.