The same place where I found the discussion of the Home Depot boycott also links a piece which maintains that the Episcopal Church is not merely guity of heresy, but apostasy:

Apostasy is a strong word. It is a word with a direct correlation with the Anti-Christ. Hence, it should not be used lightly in connection with a person, church, or denomination. 2 Thessalonians 2:3-4 says: “Let no one in any way deceive you, for it will not come (the second coming of Christ) unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction, who opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God” (NASU). The ESV translates “apostasy” as “rebellion”. Apostasy is an active rebellion against the true God (and thus, the truth) which leads to the ascension of false gods and false teachings within the heart of the Believer and within the doctrines of a church or denomination.
Therefore, to say a person or organization is guilty of apostasy is a serious accusation. But to remain silent and turn a blind eye to the truth when doing so has the potential to damage, if not destroy the spiritual life of others is even more serious. Thus, I am compelled by my conscience to speak words that may offend and anger some. That is not my intent, but I cannot remain silent when so much is at stake.
The Episcopal Church (TEC) is guilty of apostasy. The denomination is fracturing over the issue of homosexuality.

The writer apparently believes that condemnation of homosexuality goes to the very essence of Christianity — to the point where anyone or any organization which takes a different view than his biblical interpretation is not merely engaged in heresy, but in apostasy (which means the complete rejection of one’s faith.) Members of the Episcopal Church, therefore, are by his definition not Christians.
Certainly, if the Episcopal Church is no longer Christian, then neither are any other gay-friendly churches.
Well, what is Christianity, and who gets to define it?
Regardless of how anyone feels about homosexuality, since when did the biblical laws about that become such a cornerstone of the Christian faith that being gay-friendly became apostasy?
I mean, what about those who might reject other traditional rules? Like, say, the traditional penalty for apostasy:

If your very own brother, or your son or daughter, or the wife you love, or your closest friend secretly entices you, saying, “Let us go and worship other gods” (gods that neither you nor your fathers have known, 7 gods of the peoples around you, whether near or far, from one end of the land to the other), 8 do not yield to him or listen to him. Show him no pity. Do not spare him or shield him. 9 You must certainly put him to death. Your hand must be the first in putting him to death, and then the hands of all the people. 10 Stone him to death, because he tried to turn you away from the LORD your God, who brought you out of Egypt, out of the land of slavery.

Is that scriptural authority or is it not? (It’s from Deuteronomy 13:6-10.)
According to the man who charges the Episcopal Church with apostasy, the “apostasy” began with rejection of scriptural authority:

The Episcopal Church’s slow leak toward apostasy has reached a reprehensible, yet predictable blowout. And it began when the authority of Scripture was first questioned and then abandoned. It is a lesson all Christians must learn. Once the authority of Scripture has been forsaken in favor of political correctness or to allow man to do what is “right in his own eyes” (Judges 21:25 ESV) apostasy will be the result.

As to the authority of what scripture, he does not say. Does some scripture carry more authority than other scripture?
Or would that be scriptural cherry picking?