A Senate vote on the Federal Marriage Amendment is expected in early July, and the Human Rights Campaign is providing a quick, easy way for you (that is, US residents) to contact your representative(s) on this issue.
This is the letter I sent (the first paragraph is part of the form):

As your constituent, I urge you to oppose any amendment that would write discrimination against gay, lesbian, bisexual, and transgender Americans into the Constitution.
The FMA is an attempt to lend to naked bigotry the force of law. It is wrong, and vile, and I write to assure you that extraordinary circumstances will be required before I will support in any way any public figure who does not oppose it. I seldom make this statement in contacting my political representatives, as I am a meliorist by nature and experience and do not believe in delivering ultimata, or burning bridges. This is a time for exceptions, though, and I am making one. I would greatly appreciate hearing from you on this issue.

Seriously, it takes all of five minutes to send a letter; if you can do this and you don’t, I hope your asshole catches fire. Via Republic of T.