Why would it be a mistake? Marriage is not always a religious ceremony you know. Marriage through a Church for some people in Abrahamic Religions prefer marriage to be among a man and woman (according to their scriptures), but marriage doesn't have to be "religious" at all. The concept of marriage was practiced by many civilizations long before the supposed scriptural writings, which I'm assuming you're beliefs of marriage come from. Unless you believe nonreligious marriages must also follow "rules" set by certain writings as well.... 

Prior to modern society, it was one man and many women, how things have progressed.
Marriage doesn't have to be religious, it can be a civil marriage, tons of religions also have strong ties to marriage, it's strictly not a Christian thing.

So, why is Marriage between one man and one woman? Anything solid to backup that claim? Any *reason* for it to be restricted to a single man and woman?

I'm pretty sure his "back-up" has something to do with certain documents written several thousand years ago (I'm guessing the Bible due to his terminology, though even so, I'm pretty sure several men in there had multiple wives and weren't necessarily condemned for it....). This game was created with minds of the Japanese, marriage in Japan happens, and much of marriages aren't even religious. Some marriages take on traditional "shinto" styled weddings, and some bring heavily from the "western" tradition of modern weddings, chapel-y styled. Though, perhaps a reason for why it wasn't in the game to begin with, was that Same-sex marriages are actually "illegal" in Japan.

But of course, there's no "real" reason for it to be restricted. I fully agree that marriage doesn't have to be religious at all, it was actually an evolving idea throughout human history, it wasn't even always about "love" in the first place.

 
My experience is  the exact opposite on this issue, especially on the internet.  What i find especially amusing is when people like you like to act like  tolerant, open minded 'progressive' freedom fighters, and yet you never miss a chnace to openly insult, attack or disrespect Christian religion or just religious people (much like you think I did to gays in this thread) in general, like I saw you doing in that other thread about a gay teen dude in highschool. And whats really amazing is that you actually think you're any better than 'the bigots' you like to feel morally superior to and fail to see enormous irony in that

It's true that marriage was practiced before the Scriptures were wrtten. However, that doesn't matter. God intended for marriage to be between one man and one woman in the beginning, long before the first book of the Old Testament was written. Just because it wasn't recorded in the Scriptures until later does not mean that it isn't true. 

The problem is if they did include gay marriage there would still be controversy with homophobes. Either way there is a controversy.

Word use does convey attitude; yours was obvious.

Empowering people to discriminate is a terrible idea, laws need to be in place to prevent inequality, not license it.

And I am neither self-loathing, (I quite like myself) media driven, (I have a journalism postgrad and am well aware of how the media works) and as for morally bankrupt, that's rather laughable; since when does favouring fairness and equality make someone morally bankrupt? Excusing discrimination and persecuting people for their orientation, THAT is morally bankrupt.

The problem with this logic is that its basically if you don't agree with my views your bigot. Basically its the old intolerant to intolerence. The don't judge or will I will judge you for judging me. Adds nothing to discussion but drive-by comments that makes you feel you say something of worth but is actual an empty uncritical response. I hate when either side uses this as some "got you" response.

I really don't see how you couldn't change that response to agree to anything with -phobe at the end. That's why it was never a cleaver response.

I know that you weren't having a discussion with me, but I feel the need to respond to your post. 

Let me show you paragraphs 2357-2359 from the Catechism of the Catholic Church. I should also let you know that the Catholic Church is the Church started by Jesus Christ. The Catholic Church is guided by the Holy Spirit and cannot teach error. 

"Chastity and homosexuality

2357 Homosexuality refers to relations between men or between women who experience an exclusive or predominant sexual attraction toward persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained. Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity,¹⁴¹ tradition has always declared that "homosexual acts are intrinsically disordered."¹⁴² They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved.

2358 The number of men and women who have deep-seated homosexual tendencies is not negligible. This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfill God's will in their lives and, if they are Christians, to unite to the sacrifice of the Lord's Cross the difficulties they may encounter from their condition.

2359 Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection."

Where I got the Catechism paragraphs: http://www.scborromeo.org/ccc/p3s2c2a6.htm