In fifty years this will be one of the "host" of arguments that people use to try to take away tax exempt status? That's the concern? If you take away this argument, will there still be a bunch of arguments? Because that sounds like people have a problem with tax exempt status for churches, not that churches have a good reason to oppose this ruling.
What's funny -- and you likely don't and can't know this -- is that I'm just against claiming that slippery slope arguments are fallacious. I don't think, that given modern political advocacy and pressure groups, that the fallacy exists anymore. Whether we have enough in the way of protections to protect churches from marrying gay people is a legal issue. But things bleed, as a professor of mine used to say (in that sense, he was talking about, ahem, judicial philosophy...say, Cardozo, if you want) and I'd be hard-pressed not to see an advocate citing that reason for the removal of tax-exempt status. Do you? And if so, shouldn't this decision be concerning?
eta* Fifty years is a long time. Did we ever think about incorporation? Or the removal of school prayer from the public school sphere? Or the removal of nativity scenes? The innocuous decisions of one day are the benchmarks of our civilization, and I think it's time, instead of ridicule, that we recognize precedent and logical extension of precedent (as did Cardozo, to quite nicely name someone who had just been named).