A good post on the CFI site. Ron also has a link to the US Conference of Catholic Bishops' campaign, which, of course, blathers on about how the churches' constitutional rights are being violated.
That's a large and very dubious claim - based on Supreme Court precedent, there is no constitutional violation if a religious employer is simply required to comply with the same law that applies to everyone else. If it's a neutral law of general application (with some legitimate secular purpose), then everyone has to follow it, and at least as far as the US constitution goes that's okay. (There are federal and state laws in America that go further than the constitution itself in what they require in the way of accommodations of religion, but that's another matter.)