When are you not? :p

But Jason, gotta say, she's nailed it here about what separates the two. It's all about selflessness vs. selfishness/personal gratification, really.

Now, yes, I'm one to talk as a heterosexual who might never get married at the rate I'm going, and I'm far too much about personal gratification myself. But fighting to change the very core of what marriage is and what it means is a way for them to infringe on the natural and healthy institution that is family (yes, I said "healthy", so please try not to go ballistic -- two of the same sex raising a family is a bit of a freak show, period) and to open the gates to personal irresponsibility. AND, to make us all accept this as morally okay along with everything else. "You see? Look, we can get married."

Man gets guaranteed sex, woman gets guaranteed security.

There is nothing true about this statement for the general population.  I hope it's true in your family, but statistics suggest otherwise.  What guarantees sex for the man?  The woman in the relationship can refuse sex at any time, just as the man can.  If there was a guarantee, either party could force sex on the unwilling party, which inevitably can and should end up in a rape conviction.  Security is not guaranteed either, though I'd like you to elaborate on what securities you're referring to.

Women have borne the burden of risk in sexual relationships for millennia. Reliable birth control is a new development in human history. 

Yes, women are the bearers of children.  But it's not the burden you suggest, for the same reason sex isn't guaranteed - if the woman chooses otherwise, she can't be forced into having children.  Either person in the marriage can choose to use birth control without the knowledge or consent of the other.  And let's not forget, just simply refusing to have sex denies the ability to become pregnant.

 Homosexual relationships have no imbalance of risk. They have absolutely no chance of getting each other pregnant.

There are many heterosexual relationships who have no chance of getting the woman pregnant as well.  Let's start with infertility problems for either partner.  If one can't help conceive, the other loses out too, of course excluding treatment options, donors, and adoption.  When the risk of getting pregnant is gone, the balance of risk in the marriage still remains the same.

 Marriage is about family, and family is about the next generation.

And on a personal note, I find this statement insulting.  When my wife and I married, it was never about starting a family.  It was about being in love and wanting to spend our lives together.  Sure there is time to start a family later, if we choose to, but our marriage is exactly how we planned it to be.  Our DINK family couldn't be happier. 

But my head didn't explode over this; it exploded over the continually-propogated idea that gay marriage is on a moral par with, and will inevitably lead to, the kind of aberrant marriages to which Dan alluded.

1. I have conceded that that is the primal/historical basis for marriage.  But, to refer to point #3, I believe I've also adequately demonstrated that it simply isn't the case anymore.  You ask why gay people "need" marriage.  My counter-question could easily be (and is): "Given the actual roles marriage has played in our culture since, say the end of WWII (to name a meaningful if arbitrary point of reference), why does anyone need it?"  Safeguarding women and children is an explanation of its origins, but not of its current function.  Or, if that is its current function, it has failed dramatically in an overwhelming number of instances.

2. Because many people look at marriage as more than an arrangement of power.  I realize that wedding ceremonies tend to be more about pageantry than anything else, but rarely have I heard vows acknowledging a mutually-accepted balance of power.  There is something else going on, perhaps a little different for every couple, but I think that, now at least, it has as much to do with entering into a new phase of societal expectation than the primitive distributions of power/protection that your trying to get me to accept are still part and parcel of marriage in 2009.

3. I'm glad to see I can, in fact, prove something.  Of course, I don't see gay marriage as "destroying" anything.  I do see the failure of heterosexuality to widely maintain the sanctity of marriage as an obvious rebuttal to the idea that allowing homosexual couples access to it will somehow besmirch this heretofore pure-as-the-driven-snow institution.

I'm going to take your anecdotal evidence with a grain of salt, and also suggest that Augusten Burroughs has only slightly more autobiographical integrity than James Frey.  Suffice to say, I would need to see actual, sociological/psychological studies before I'd begin to accept the premise that homosexuality is, as you imply, generally a function of abuse.  To begin with, it defies both common sense and basic precepts of modern psychology.  A person molested as a child by a person of the same gender would be more likely to repress the incident and its ramifications and overcompensate either through asexuality or hyperheterosexuality.

The fact that NAMBLA exists should give anyone pause, but I don't see the connection.

Pointing out that the slippery slope is an argumentative fallacy is crucial to this debate because it is so often used to associate homosexuality on a moral par with far more insidious and destructive behaviors that do not involve consent whether matrimonial or just sexual.  As to "why it's needed," I've pointed out over and over again that a) It's a matter of social equality and b) Any objection to it lacks, almost by definition, a material concern. 

No.  Because what "they" may or may not ask for next is not relevant to what it is right to legalize now.  To say otherwise would be to cling to the slippery slope fallacy like a tree in a hurricane.

As you may or may not have noticed over the years, Shy, this debate is one upon which I will not concede too much ground.