The scenario with a bag of bagels isn't one of the ones causing an issue, so as an example it doesn't apply. That type of transaction is fine - that the buyer is gay or whatever is irrelevant, it has no bearing on the transaction, it has no reason to come up. It'd be quite a stretch to say that by selling them some bagels you are endorsing them in any way.
The issue is with scenarios when the sexual orientation *is* relevant in the transaction and where participating in it can be interpreted as endorsement. Should someone be able to politely decline to participate in that transaction? It seems reasonable to allow it, that's all. What if it's a KKK rally? Or a Nazi rally? It could be any number of things that one person finds objectionable and another doesn't. Is the concept of civil liberties so completely eroded that we balk at people being free to choose whether or not they participate in such a thing?
So a more relevant example would be the wedding photographer. This isn't someone who is trying to shut down the gay wedding, they are not trying to make a spectacle of things. They would simply prefer to not participate and reserve the right to let someone else have that business.
It seems wildly unconstitutional and not a little ominous for the government to swoop in and *force* that photographer to be a part of it. If anyone's rights are being infringed in that situation, the photographer has at *least* as much claim on infringed rights as the people getting married.
Obviously, people can disagree with the photographer and boycott his services if they want and let market forces either keep him in or put him out of business. That's fine - he's not asking for any special treatment under the law, so if he goes out of business, so be it. That we're even talking about it being ok for the government to force him to enter into transactions seems ludicrous.