Hi Damien,
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Sorry, but I think it's a mischaracterization to call most of those comments transphobic.
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It wasn't particularly clear at the outset how 'out' you are exactly to the family. People were concerned that perhaps you either weren't out, or that you were only out as gay....with your family having no knowledge of you being trans...and it coming as a big surprise at the wedding.
A lack of knowledge of the situation can cause some misunderstandings.
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The concern was about showing respect to the people that invited you. About not distracting from their day with a big reveal that they and your other relatives would be blindsided by. I personally don't think you should go as anything but yourself. But if this was the first time they were all finding out about it...in deference to the marrying couple...overshadowing their happy day with your news would not be appropriate or polite to the courteous invitation they extended to you. (This is generally expected of any big news...not just being trans. Basic rule of thumb: Don't steal their thunder. It's considered very rude.)
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I have a similar situation coming up next year. I was invited to not only be at my cousin's wedding, but she wanted me to be the photographer for the event too. She doesn't know I'm trans. She doesn't know that I'm on hormones. A year from now, people are going to notice the changes...it can't be helped. I'll probably be out by then too. I made up an excuse as to why I couldn't do it for her, because it would have been very disruptive to her wedding. People should be focusing on the married couple...not rubbernecking at the photographer and whispering among themselves about something they find surprising/shocking. Even though I wanted to help her out, it would have been very rude of me to accept her request knowing what's coming next year. Showing up with a "Surprise! This is me. BTW how do you like my new dress? Let's take some pictures!" situation, would have been terribly inconsiderate of me.
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If I'm out by then, and I probably will be...and my cousin wants to invite the *real* me that she doesn't know yet...then I'll come to the wedding. Right now, she only invited the empty shell of a man that she knows. If I'm out...and I can't go as me though...then I won't go at all. It's her day...her invitation...her decision on who comes and who doesn't. It's her party, and as the hostess...regardless of whether I like or agree with her decision...she deserves at least that much respect.
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Someday I won't have to make fake excuses. Someday, I'll be able to be myself 24/7, and people will know me for me, and invite the woman they expect to come. Someday showing up in a dress won't be shocking to my relatives. Yes, I deserve to be shown respect for who I am. But respect is a two way street. And next year, I will have to respect her decision, just as she will have to knowingly respect who I am if she wants me there.
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Honestly, it sounds like it shouldn't be much of an issue for you at the wedding anyway. Even if you weren't completely out as a man, and were only out to your relatives as a 'butch lesbian', going in a suit isn't much of a stretch in the realm of acceptability. (Heck, my coworker wore a suit to her own wedding.) Me going in a dress if people thought I was gay would be another matter completely though.

As long as respect and consideration is being shown both ways, there shouldn't be a problem.
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Anyway, I hope you enjoy the wedding,
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Kay