The Question (and Statement):
➢ Considering there are progressive Orthodox Christians that exist and support same-sex marriage and transitioning, how can you say that Orthodoxy holds to this position?
➣ Not all Orthodox hold the position that it is sin.
The Answer:
I am aware of these individuals. I know of groups such as orthodoxandgay.com and similar associations professing to be Orthodox. I also know of Protestant evangelical churches that are very accepting and supportive of these acts, just as I am aware of some that conducted a George Floyd communion.
Despite these positions, especially in light of the Orthodox tradition that has endured over centuries, I do not consider such views informed or consistent with true Orthodoxy. None of the Church Fathers ever endorsed the misuse of matrimony, and historically, they prescribed penalties for wrongful practices which, in many cases, the contemporary Church no longer enforces.
Even Patriarch Bartholomew, often cited as a progressive voice within Orthodoxy, condemned such unions. On September 7, 2013, at the Holy Cathedral of St. Symeon in Tallinn, Estonia, he declared in his homily:
"To our Lord Jesus Christ, who blessed families through the Mystery of Marriage at Cana of Galilee and changed water into wine, that is, into joy and feasting, and to His Body, the Orthodox Church, the partnering of the same sex is unknown and condemned, and they condemn the contemporary invention of 'mutual cohabitation', which is the result of sin and not the law of joy" (Bartholomew, Ecumenical Patriarch Bartholomew Addresses the Issue of Homosexual Marriage, trans. John Sandopolous).
He continued:
"The Church, my beloved parents and children, and subsequently the family, which consists lawfully and by the command of God of men and women, and the children acquired, is not a foundation or association or a simple organization, but a Body, as it is wonderfully depicted by the Apostle Paul. And this parallelism is accurate and true. Church and marriage. Husband and wife. Body and its members. This community, signified in the Mysteries and in the obedience of Faith, both in the Church and in the family, is sanctified and mystagogued through the Mystery of Marriage, which, according to the Fathers, is a mystery of co-creation, and the ontological link of love with the Head of the Body, to ensure health and life, which is salvation and sanctification" (Bartholomew, trans. Sandopolous).
Individuals professing Christianity while in error, or Christians supporting these unions out of ignorance, do not influence the standard, enforced, and upheld doctrines of the Church. They are in error and advocate positions contrary to God. It is comparable to looking at the LGBT+ community and citing the views of certain homosexual individuals against transgenderism as representing the entire community. It does not.
The Church is not built on erroneous positions, nor on the beliefs of churches outside her communion. When discussing the positions of the Orthodox Church, one must reference the Church itself, her history, practice, and theology, guided by Sacred Scripture and Holy Tradition. Protestant or Roman Catholic positions, while worthy of discussion in ecumenical contexts, do not define Orthodox doctrine.
Reference
Bartholomew. Ecumenical Patriarch Bartholomew Addresses the Issue of Homosexual Marriage. Translated by John Sandopolous.

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LGBT+ and the Church
SpiritualThis book will be addressing the LGBT+ issues and the Church. The primary chapters will discuss God's design, the sin and reasoning, and more, and will eventually begin going into a question series. In the question segment, it will address questions...