1. A significant number of conservatives will depart, accelerating the steady membership decline of the mainline Protestant denomination -- which, according to its own record-keeping, has lost 10 percent of its membership since 1987. This is a dying church, not a growing church, and this departure from Scripture and tradition will only make things worse.
2. Many conservatives may depart, making the short term situation worse, but the ELCA is on the right side of history. Younger people are dramatically more accepting than their elders of same-sex marriage. As these people age, start families, and go looking for a church, they're going to want to affiliate with a church that accepts gays fully. There will the ELCA be. Long-term, these Lutherans will benefit from their decision.
3. Many conservatives will depart, further weakening the declining denomination. But those who remain and who anticipate a rebound because of demographic eventualities will be disappointed, but not because conservatives will post absolute gains in membership. Rather, overall church attendance will continue to decline slowly as more Americans identify as secular. America will track the European model somewhat: conservative churches/denominations will prosper relative to liberal ones, because they offer a clear alternative to mainstream culture. In other words, the Christians who stay active in churches will tend to be those who are more highly motivated to affiliate with churches that offer a clear alternative -- but overall, secularization will continue steadily, with fewer Americans interested in church at all, and the gay-clergy decision won't have proved to have mattered much in the overall scheme of things.
Unfortunately, I agree with Dreher that #3 is the most likely scenario. It's not what I'd like to see happen, but it's what I think will happen. Religious faith, at least the Christian sort, is simply becoming less and less relevant to an affluent, pleasure-oreiented society that has learned to live by its feelings, gratify its desires, and reject anything that would impose restraints on its appetites and inclinations.RLC