First: most of Draug's answers are really good, and i'm just echoing them.
"it's possible they could severely reduce the number of gay people.
Whether or not that's a good thing is another question."
This is called Eugenics, and is considered a bad thing, mostly lost popularity because of the Nazis, and the fact that nobody likes to be compared with the Nazis...
Notice that abortions based on gender happen in the two most populous nations in the world (India and China) and are causing untold social damage.
Also worth noting, deaf communities in the developed world are being destroyed by new technology which gives kids partial hearing, the state of being deaf - deafhood - is being destroyed and their culture wiped out. This practice may be described as cultural genocide.
"Alternate question: Why is *government acknowledged* gay marriage good for society?
And don't play the "It's only fair" card"
Because dividing people and excluding them from society makes them fight, and conflict decreases co-operation which is bad for society (see: theft, murder, war, union strikes, terrorism etc) - oh wait, that is the fucking 'its only fair card' because guess what dickhead, making society fair is a good thing for society.
'For example, government don't equally tax people' - eh, yeah they do, everyone gets the same formula applied to them - your first €16K is tax free (poverty level), then your next €10K is taxed at 5%, then your next €40K is taxed at 23%... etc
"And, gays can currently get married - to a member of the opposite sex, just like heterosexuals, right? " - and no, almost no person in the world wants to marry a member of the opposite sex (except to get a green card, or EU citizenship... or whatever exploit) With these exceptions, people want to marry someone they love, note: you don't marry a randomly selected member of the opposite sex, you choose to marry a person you love - that is the criteria people use, and that is the perspective we should consider. To look at it otherwise is disingenuous.
"If gays can get married, then I want to marry my girlfriend and have two wives - I'm being disenfranchised from my special three-way love and our lover is oppressed by this bigotry - she's being cut out of rights of visitation when I die, etc even though she shares our bed, blah blah blah." - yes, this is a valid objection to the monogamy which dominates marriage, there is no particular reason i can think of to prevent polyamourous couples from having group marriages. Please read 'The Ethical Slut' if you wish to understand a bit more about this life-style.
"Government disenfranchises people all the time, but people throw this argument out as if it's the end of the discussion in gay marriage. Logically, it's not...not even close."
The fact that government fails many times over to make everyone happy, doesn't mean it can't or shouldn't seek to improve.
That's like arguing that there will always be poverty, therefore we shouldn't bother with any charity... Just because there will always be problems doesn't mean we shouldn't attempt to fix some issues.
"Draug - why does it need to be "marriage"? Why can't it be a simply "civil" arrangement that bestows upon them contractual rights to hospital visitation, financial benefits, etc?"
It doesn't, but that the 'treating equally' clause means removing the word marriage from all currently married couples. Which politicians don't have the backbone to propose.
"YOU, personally, have the God-Given right of your creation TO PERSONALLY PURSUE HAPPINESS."
Which includes the right to pursue marriage equality, and legislate for it.
@Draug: "And the only reason I restrict it to two people is for logistics purposes. If 8 people are legally unified and one of them is comatose, how do you decide which opinion to follow when the other 7 can't agree?" - what if one parent has 7 children, and these next of kin can't agree? This kind of logistic problem is not prevented and currently (in Ireland at least) most doctors will choose a course of action based on medical grounds, not family decisions.
"Marriage is a label that is sacred to some" - whoop-de-fucking-do; scientologists may consider psychiatry to be a sacred (but evil) label, and I see no reason why their religious beliefs should dictate to the state what they should or shouldn't call anything.
"They push for the cultural add-on of calling a gay civil union a marriage, even though throughout human history it has never been so." - that is not true, marriage was considered a personal decision, churches only got involved as a matter of public record - because they were the only ones who could write things down and keep the records.
The Christian cultural attacks on gay-marriage have been around for less than 2 thousand of years, but no more than that. And male-male relationships were common in pre-christian greece and rome (which we have writings of, whether they were common elsewhere is harder to state) AND there is at least one recorded gay-marriage in a monastery in Spain in 1061, see : http://www.galiciae.com/nova/78210.html
"The Federal Government should not be involved in making cultural decisions." - agreed, it should be about implementing cultural decisions made by the people. As it is doing here.
"This isn't about equal rights, it's about attacking the existing majority cultural; it's about cultural change, period." - This IS about equal rights. It is PRECISELY about cultural change as the only means to achieve those rights.