Dear Rob,
Why do you want to ruin my wedding?
I bet you didn’t know this but at least 0.5% of the reason why Anna and I wanted to have a wedding is just so we could meet you. You wouldn’t want to crush our delicate lesbian hearts just because you couldn’t be bothered to buy a suit, do you?
I have lots of brilliant reasons why it’s so important that you be present for me and Anna’s wedding. Want to hear them? Well, too bad. Here we go:
First of all, we’re beginning to think you don’t exist. I’ve known Vernon since I started working in the Office of Admissions in 2002. It’s 2011, Rob. You do the math. Did you do it? So now you know that Vernon and I have known each other for 9 years and I have never met you. NINE years, Rob. 9 years. I’m beginning to think you are some sort of mythical creature like the Easter Bunny…

- George W Bush LOVES the Easter Bunny
Or Samsquanch…

Rob, you can tell everyone what a "Samsquanch" is when you come to the wedding.
Or Marriage Equality.
Secondly, I think you and I would be great friends. Vernon and Anna Saraceno are great friends. Vernon even has a shrine to Anna in his office. Do you have a shrine to me in your office, Rob? I didn’t think so. If we were friends, we could talk about all sorts of stuff like… well, I don’t know what. But I know you could help me figure out more ways to pester Vernon. For example, I didn’t know your refusal to buy a suit would annoy him so much. I’m sure I can get some mileage out of that one!

Some might call this "creepy"; I just call it "love".
And finally, I have no idea what you look like. I hear you and Vernon look alike. Anna and I have the same name but we look nothing alike. You and Vernon have completely different names, but look like each other. Rob, don’t you see how these are the makings of the most awesome, queerest friendship quartet that the world has ever seen?! Why are you foresaking our destiny?!

This is the only picture of Rob I have ever seen.
So, Rob, yesterday Vernon told me that he might not let you come to the wedding if you don’t buy a suit. This troubles me, Rob. The wedding is in 22 days. 22 days, Rob. That means you have THREE WEEKS to find the suit of your dreams and get it altered so that it fits you like a glove. Do you really want me and Anna to only be 99.5% happy on our wedding day? This doesn’t bode well for the future of our non-existent friendship, Rob. Your refusal to buy the appropriate apparrel for the happiest day of my life makes me feel like this: 
You don’t really want me to feel that way, do you Rob?
To aid in your search for the perfect suit, I have done some internet searching for you. You can even buy a suit on Amazon.com, Rob. Who doesn’t love shopping on Amazon? I know my future BFF likes shopping on Amazon (in case you haven’t realized it yet, my future BFF is you Rob. It’s always been you.)
In conclusion, I hope my honest and heartfelt confessions have inspired you to go out there and find the perfect suit for what will surely be the most fun, least legal wedding you attend all year. You’d better be there Rob. I want to be 100% happy on my wedding day and only YOU can make that happen.
One more thing, Rob. No Texas Tuxedo, okay? It might have worked for Justin and Britney but it ain’t gonna work for you.

What's their excuse? "It was the late 90's"?
Sincerely-
Your dearest friend that you just haven’t met yet
[To the rest of our friends and family: By now I'm sure you agree with me that Rob just needs to get off his duff and buy a suit for our wedding. Rob, get it together man.]