This is one you can have some fun with.?

Seriously, the correct term is "Honor Attendant." That is the term for someone who is standing up for the bride or groom and is not specified by gender. These days it is common, for instance, if there is re-marriage the bride might want her teenage son to be her Honor Attendant instead of having a Maid of Honor or a daughter could be her father's Honor Attendant instead of him having a Best Man, or the bride or groom might have a good friend of the opposite that they want in their wedding party. I kinda like "Fairy Odd Guys" though!