This is something I posted in another thread about homosexuality. I think it is relevant here... The key is to take this from within Orthodox ecclesiology - which is, to say the least, NOT egalitarian nor ecumenical, but furthermore is agnostic as to the fate / validity of those outside her communion (with the exception of those who directly schism from her).
As for the Catholic poster who challenged why our interpretations of the Bible had validity against that of a KKK member - we stand in the tradition on both counts. Homosexuality is traditionally condemned as a sin, whereas being racially different traditionally is NOT. Therefore, the criterion of traditional understandings of scripture holding precedence in the Orthodox Church is being consistently applied. We aren't being hypocritical - we're being true to our approach to theology.
As I am an untrained theologian my reply will be somewhat speculative. That is to say - it makes sense to me, but I'm not confident enough to say that it is "the" Orthodox reply to this question. My conclusions are Orthodox, however, and perhaps that is in some way sufficient.
Our theology of marriage begins, as all things do, with the Holy Trinity. We profess that God is love in His essence - love within Himself. This, however, cannot be a narcissistic sort of selfish love of one monad unto Himself. Rather, as we are called to imitate that love, we know that love to be other-centric and self-denying. This is manifested in the great mystery of the Triune God. He is Father loving Son loving Spirit. While there is hierarchy in the relationships (Father as fountainhead of divinity) they are co-equal and of the same nature, fully God. One God, Three Persons, Perfect Hierarchy, Perfect Love.
We must understand this love to have any hope of understanding human love. We were made for relationship - specifically relationship which is ascetic (that is, teaches us to be other-centered and thus helps break down our own ego). Furthermore, we were made for relationships that, while having hierarchy, are so perfect in their love that the hierarchy fades to the background and we see functional (and true, thanks to the perfection of this love) equality. Human love can never achieve the fullness of union that the Divine Love has by essence, but we can begin to imitate it. This is what it means for us to become one with one another. Marriage is the divine sacrament in which these principles of love find fulfillment as an icon of the Divine Love.
There is another divine relationship, however, to which marriage is also an icon: that of Christ to His Church. Ephesians directly relates that to the marriage. We have, as prophecy, the words in Genesis that speak of a man leaving His father (in heaven) and His mother (on earth) to join with His wife that the two may become one flesh. This certainly speaks of earthly marriage, but you can see how it prophecies and finds fulfillment in Christ.
At what point do we understand ourselves to have become one flesh with our Lord and Bridegroom? Well, He became man in the Incarnation, joining the divine and human natures so that we may participate in His divine energies. We also enter, iconically, into His death and resurrection (and thus the energies of His resurrection - His saving grace) by baptism. We recieve the grace of His forgiving hand in confession. We recieve the Spirit - even as He had it (as a man) - in our personal Pentecost: Chrismation. Chrismation makes us into "little annointed ones" - we become little Christs. Those who are ordained become icons of Christ in His role as priest, revealing in the liturgy the things of heaven, serving the faithful as Christ served His disciples.
The fullness of our oneness with Christ, however, in this lifetime, is fulfilled in the eucharist. This is the eros love of this wedding between Christ and Church - the foretaste and full realization (for those who have eyes to see) of the feast of the bridegroom. Certainly we look forward to the 2nd coming, but the Kingdom of Heaven is at hand today - Christ came to save this world - and the eucharist is our full participation in that. Here, our Lord comes to us in the flesh, and we take the body of our Lord into ourselves, and we become one flesh with Him. If there is no physical union, there is no marriage (hence why we still call Mary the bride unwedded). We are Christ's bride. We are one with Him.
Marriage is, therefore, an icon of this love. Here we have the man, who is to be like Christ to His wife - so loving of her as to give the totality of his life to her benefit. He is to give her all his dreams, all his hopes. To make his life about her life. To serve and not to demand. To die to himself. This is to be done no matter what she does in return, for if we demanded that our wives reciprocated we would be hypocrites. Christ Himself has the worst bride possible - think of all the sins we commit! We must thank God that He has provided us with such a model of a husband, for we, as a bride, must be a hideous harlot and adultress, yet He dies for us.
Here also we have the woman - the one who recieves the body of her lord in that act of vulnerability and love, who embraces the totality of who he is and submits to him. That is both a physical and emotional recieving, embracing, and submitting. Here we have hierarchy, but hierarchy which, when lived out with the love that is the Divine Love, fades into non-existence and becomes perfect love.
Here is one level on which homosexuality is "wrong" (or spiritually unhealthy / not-an-icon-of-the-kingdom). Two men cannot represent Christ and the Church. The Church is uniquely feminine. To take that from her is to distort the image. Furthermore, Christ is uniquely male (in so much that He became a boy), and though God is in essence gender neutral (by human standards) within the context of this typology, God is the male. Two males or two females distorts the image and is, therefore, not the sacrament of marriage. With no sacrament, there is to be no sex. Therefore, since the Church cannot marry two men or two women (as they don't form the correct and given icon of Christ and the Church) two men or two women ought not to have sex outside of that marriage.
Another level exists to this, though. The eucharist is not just unitive in nature. It is life producing. We have Christ born in us by the grace of the sacrament, and by that birth we become more Christ like. In this, again, we have a woman as our model (for in so much as we are the Church, we are feminine). Mary, the Theotokos, is our model and hope. She, by the "yes" she gave to the incarnation, became one with the Holy Spirit, and by that union and at the good will of the Father, she bore the Son physically in her womb. She BORE Christ and had Christ BORN from her. By this union, she became more Christ like. She is truly the icon of the bride of God! In this way, she is an icon of the Church.
Homosexual union cannot produce life - not naturally, anyway - but heterosexual union (generally) can. It fulfills the eucharistic Christ-Church icon to its fullest. And by giving birth to children (the prayers for which, by the way, are all over the marriage sacrament - most who see an Orthodox wedding are suprised by how much focus is paid to having children) we learn, again, to be self-sacrificing in our love. It becomes about the child, not about 'me.' This again, goes back to our understanding of Divine Love within the Trinity. Because homosexual union can never produce such a miracle, it cannot recieve the prayers of the sacrament of marriage and, once again, becomes sex outside of marriage.
Some occasionally bring up those who cannot have children from within hetero-sexual relationships. The first point about how man-woman = Christ-Church still suffices on its own, however, I also accept the idea of the miraculous births God accomplished for the infirtile. It fits so perfectly into this typological understanding. Who is it that produces life in us? Well, it may be explanable biologically, but ultimately it is a miracle of God. The "miraculous" births remind us of that. Keep in mind that Mary was celibate, yet was a perfect icon of marriage to God. There are celibate men and women in the Church for whom she is an icon of their life (along with John the Baptist and Christ Himself). Some marriages choose to live as brother and sister (St. John of Krondstadt did this).
I think the principle is this: the marriage sacrament, if we are going to make general rules about it, cannot be between man and man or woman and woman because no possibility of producing new life exists. Because it is possible (even if unlikely) between man and woman, the church can marry them. Beyond that, it is up to God.
Hope that helps!
In Christ,
Macarius