Daughter of the groom’s role in father’s second marriage

A little background:

I am 26 years old female, and I have two brothers (ages 20 and 24). My parents got divorced when my youngest brother was 18 (about 2.5 years ago in May 2010). My dad started dating again pretty soon after the divorce, met his fiance in Feb 2011, and is set to marry mid-May (about 4 weeks from now). I’ve only met his fiance twice, once when I went back to visit him for Thanksgiving in Nov 2012 (we live 1000 miles apart) and once when they came to visit me in Feb 2013. Although it has been hard, I have been very supportive of my father’s new relationship and more than tolerant of his wife-to-be.

That being said, there have been some issues regarding the upcoming wedding.

I found out “accidentally” (when asking what sort of dress code was expected at the wedding) that my dad had asked my two brothers to be part of his wedding party (the “groomsmen” as he calls it). He had never mentioned this to me, and it seems like it would likely have been kept a secret right up until I arrived at the wedding had he not accidentally mentioned it when discussing attire. I was immediately alarmed and hurt that he did not ask me to be a part of the wedding as well. It felt to me that he was intentionally leaving one child out because I am not the same sex as the parent getting remarried. I brought it up with him (tearfully) over the phone. He was confused as to why I would want to be part of the wedding party at all and didn’t feel like there was a place for me on his side of the altar because that’s where the groomsmen are supposed to be. After talking about it with his fiance, they decided that I could be part of the wedding, but I’d have to stand on her side as a bridesmaid. I told him this was not something I was comfortable with, and that if this was my only option I did not want to be apart of the wedding.

I’m not trying to make a wedding that is not about me, all about me. I just don’t understand how my dad could leave one of his children out (based solely on the fact that she is his daughter and not a son) and then when they finally agreed to include me it’s only as a bridesmaid. I understand that he wants to have a “traditional” wedding, but there has to be some sort of precedent that I can show him that exemplifies that even if it is “traditional” it’s not comfortable for any party to have the groom’s daughter on side of the new wife she barely knows. Is it really that common/uncommon for daughters to stand up on their father’s sides of the altar when he remarries? And if I am totally off-base, I guess I’d like to know that too. It’s starting to feel like this wedding is ruining the relationship I have with my father.