I believe the Unitarian growth is due to their acceptance of the homosexual lifestyle and homosexual unions, and homosexual clergy ordination.
Episcopalians, the ELCA, the PCUSA and the UCC also have that, as well as the Christian Church / Disciples of Christ, and the American Baptist Convention.
Yet all of them are shrinking.
The only mainline church that doesn't officially have gay marriage is the UMC, but UMC clergy just disobey the rulings of their General Conference and the district superintendents and bishops let them get away with it, on the grounds that the people voting against gay marriage are from Africa and their opinions are therefore not relevant to the UMC in the US.
I find it really appalling.
There is even talk of schism.
The UMC in Africa remains traditional, and conservative, and is growing. My late uncle, may his memory memory eternal, was a UMC missionary tortured by Salazar in Angola in 1957.
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However, because the UCC, et cetera, have gay marriage, but are shrinking fast, I think that rules out the idea that gay marriage is causing the Unitarians to grow. Rather, I think the growth of the Unitarians is driven by their rejection of the Gospel in favor of a doctrine-free religious pluralism.
All of the mainline churches still preach the Gospel, even if they accept gay marriage and homosexuality, and I am of the opinion that the incompatibility between the two produces cognitive dissonance. Cognitive dissonance is known to make people physically uncomfortable, and directly explains the dwindling size of the congregations.
I would also note even in the ECUSA, there are conservative bishops, conservative dioceses, conservative congregations and conservative priests, which are healthy and functional. It's the moderate churches which are sinking fast. A few of the ultra-modernist emergent parishes like St. Gregory of Nyassa in San Francisco have carved out a niche, but I am of the opinion that the demand for such parishes is limited; it is clearly not enough to sustain the church as a whole.
In New York City, the Cathedral of St. John the Divine, which recently installed an altar depicting Jesus as a woman, and which apparently has some really frightening looking Halloween parties (I posted a thread on it in Denomination Specific Theology), is running out of money and is having to sell off their beautiful surrounding parklands to commercial developers.
Holy Trinity on Wall Street on the other hand is a bit more traditional, and also is the wealthiest parish church (I think of any denomination) in North America, with priceless endowments of land and other assets.
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Because gay marriage and the reading of the Gospel produce cognitive dissonance, the attendance at mainline churches will fall until they return to the traditional Christian faith, which historically, I think most of them did a very good job proclaiming. The Unitarians will continue to grow, because having discarded the Gospel first through the rejection of the deity of Jesus Christ our Lord, and secondly and more severely, by embracing the transcendtalism promoted by the likes of Rev. Ralph Waldo Emerson, they have eliminated from their churches the actual Gospel message, so there is no cognitive dissonance owing from a clash of ideas.
The Gospel convicts people of sin; homosexuality is a sin, and when a minister reads from the Revised Common Lectionary, and then delivers a sermon that might, for example, praise gay marriage, the result will make most people very uncomfortable.
