This gets at something really interesting, and perhaps this is where the line is drawn. Drawing the picture of Mohammed is a sin in the Muslim faith, so to make a cake with his face on it is to commit the sin yourself (assuming that you are a devout Muslim baker, of course). This is definitely a violation of the First Amendment for the government to compel you to do this, and that definitely makes sense.
For the devout Christian baker who views gay marriage as something that runs contrary to his faith, the issue certainly becomes cloudier. For this baker, he has to argue that making the cake makes him complicit in or enables a sin that violates his faith. Committing the act and enabling the act are obviously different things, so arguing them as the same thing is a flawed premise. Of course, this doesn't answer the question of where the line should be drawn by the government. Is enabling the sin a violation of the First Amendment, or is only committing the act itself something that you are protected from? I think reasonable people can disagree on that answer. Personally, even though I disagree with the baker in the CO case (I think he should have just made the damn cake instead of draw a hard line in the sand, which resulted in going out of business), I think that such a baker should be protected from enabling the act, but that's probably what you might expect from someone with more libertarian beliefs. I certainly understand if you draw the line in a different place in accordance with your own judgment and system of beliefs. Where I break ranks with most of the other folks in this thread arguing on behalf of the baker is their reasoning behind it. I think many are arguing that committing and enabling an act are the same thing, and as I've laid out, that is a fundamentally flawed argument.
The question about where to draw the line is different federally vs. at the state level too. According to what I've heard and interpreted (yeah this whole post gets the ol' IANAL disclaimer), the question about what is a "real" sin is not a very important question at the federal level, and the enabling question isn't very important either, since personal religious beliefs are protected so long as they aren't just ad hoc nonsense. In this case, while we "know" that the baker's objection is crap, it's not entirely without reason. So the court seems obligated to accept it as a real reason (part of why they ruled against Colorado is that they were cynical about the baker's intentions). The only thing that can trump that would be discrimination against a protected class, like race. But since sexual orientation isn't protected federally, the religious belief wins.
In Colorado, the sexual orientation and the religious belief hold the same weight, so it's an easier question for the state. The only balancing act they have to do is to weigh the discrimination in public accommodation against the religious objection, which in this case is a trivial thing to determine. That's where I think the "real" sin comes into play- it matters if it's really a sin, or just something the baker doesn't want to do. They just have to ensure that they are actually considering the religious objection fairly, and not being religiously discriminatory themselves, at which they failed big time.
Given all of that, the Supreme Court can then weigh out the rights of the state vs the first amendment. Even if the court might weigh things differently on the discrimination itself, the court has to consider allowing the state law to stand, because it's okay for states to offer more protection than the federal government- it's not unconstitutional to protect sexual orientation. The state should win on that point. And the last thing to consider, which is where a lot of the media and expert speculation was located, is whether making the cake is a religious expression. I think most reasonable people agree that's pushing religious expression too far, because now we're talking about basically denying any gay person any custom service in any public shop if it the customization even hints at the idea that gayness exists.
The idea about enabling and committing acts being equivalent is some hellaciously tricky and destructive dogma. It has its roots in sexual and material coveting. Our natural emotions are considered sinful in and of themselves, such that many mind-chained followers are truly convinced that they are sinful for their jealousy or their desire. Ironically, that
is a deeply held and sincere religious belief. Oh, religion, you card!