The very first letter

Let's get started with the first letter.  I imagine the author of this letter read it over several times, and thought "This is irrefutable.  Surely, Congress will legislate the gay out of these sinners!"  I will give the author credit by saying that it is comprehensible, which is a feat for someone who has views that were commonplace in a time when education wasn't considered necessary.

Whoo, boy!  This guy has opinions!  Let's talk about them.

Please honor cultural history, biological reality, people of faith, etc...
Let's honor these very broad and subjective categories by imposing our very narrow and simple world views upon them.

It will never be equivalent, it is dishonest, blah blah blah...
This is an odd bullet point, because it doesn't seem to be a complete thought to me.  What is a lie?  Gay marriage is a lie?

Traditional marriage has been honored for millennia in all cultures as a unique institution...
Nice try, but this is not a fact.  It isn't even close.  MOST cultures honor some form of marriage, but what is considered traditional is not the same across the board.  We know from historical texts that marriage is defined differently based on several different aspects of a society's culture.  There are cultures who have no recognizable form of marriage at all.  Furthermore, there are cultures who don't subscribe to traditional gender assignments.  There were Native American tribes that had 3 genders:  Male, Female, and a special Cross-gender of people who identified as the opposite gender of their biological sex.

Only Traditional Marriage contains the biological potential to produce children and familes and to continue to propagate the species...
I will cede the point that YOUR version of traditional marriage is the only one capable of pro-creation.  I don't care, but you're correct.  Your version of traditional marriage is also the only one capable of creating homosexual children, so perhaps you should stop breeding if that worries you so much.  And just so we're clear, the human race is having no difficulty "propagating the species."  Overpopulation will soon be a real threat to our planet.

The world's great faiths consider homosexuality wrong.  The Bible and Torah both, respected by Christians, Jews, and Muslims blah blah blah...
I didn't realize that Muslims were big fans of the Torah.  I like how you conveniently left out the Quran, probably because you don't consider it a holy book, since those Muslims will surely be going to hell.  While I can't say too much about the Torah, I have read the New Testament of the Bible.  I can say with a reasonable degree of certainty that Jesus was not concerned with sexual preference, considering he never once mentioned it.  Instead he chose to focus on ideas like "love your neighbor as thyself," and "Judge not, lest ye be judged."  People who focus on the ancient Judaic laws from the Old Testament have missed the point about Christ and what he stood for.

Gay couples who are willing to commit to an enduring, permanent relationship deserve the same civil rights as a man and a woman willing to do the same.  But that is the role of a Civil Union...blah blah
"Here let me show you that I'm not a bigot!"  We get it.  You don't want us to think that you are a hateful person. However, you contradict your own point within the same bullet by saying that they deserve the same legal rights, but they don't deserve to adopt.  I didn't realize civil rights were granted circumstantially.  Also, there are some huge differences between Civil Union and Marriage.  They are not equivalent for legal purposes.  Marriage is recognized federally, and by governments worldwide, whereas civil unions were created by States to grant legal rights at ONLY A STATE LEVEL.  The word "marriage" also carries an extremely powerful connotation that is not given to "civil union."  There is a very real attitude that somehow a marriage is more important and valid than a civil union.

Whether you believe in Father God or Mother Nature gay marriage will never be equivalent to traditional marriage between one man and one woman...
Again, you are mixing up what you believe to be "traditional" and the opinions of the rest of the world.  Foot binding is traditional in China.  By your logic foot binding should be required for all women because it is traditional.

Science supports...children do best when raised by a father and mother.  There is NO science documenting how children of same-sex couples will fare in general.
This is lazy, and dishonest.  Science certainly does support the fact that children do better with a father and a mother.  As opposed to just a father, or just a mother.  Science does not conclude that a father and a mother is better than two fathers or two mothers.  You are correct that there isn't a lot of evidence of how children of same-sex couples will fare.  Perhaps that is because of lack of sample couples.  Up until recently, this issue was unheard of.  Maybe it's time to perform those studies now that the world has progressed enough to allow them.  I'm nearly positive you will not be pleased with the results.

This issue is too important to be left to a vote in a General Assembly, Senate or House.  This issue should be placed on the ballot of a general election.
 I really don't think you'll like the results.  According to polls Illinois voters are just dandy with gay marriage.  So your final parting line of "I believe a vast majority of your constituents feel the same" is especially hilarious to me.  Not only do you believe in all of this, but you also mistakenly believe a vast majority of people agree.  

Unfortunately, this letter isn't one of a kind.  These opinions are out there, and they come from our classmates, colleagues, parents and grandparents, and so forth.  Luckily, these people are being outnumbered, and even within our lifetimes may be a very small minority.  While it isn't a solution to simply wait for progress, I believe it is important to let these people know that they are a dying breed, and that progress is coming regardless of how many letters they write.