Of course they can tell whoever they want. It becomes a smear campaign when the gay person sues the Christian business for every penny and avows to punish anyone with beliefs different from their own.
My friend, the Christian in this case believes that gay marriage is offensive to God and a grave sin. That is their belief. It is not the government's job to decide what a Christian does or does not believe. By arranging flowers for the wedding, yes, many Christians would believe that would be participating in what they consider evil. Would you force a Muslim to eat meat on their holy days? Of course not. For the same reason a Christian shouldn't be forced to act against their beliefs.
Forcing a Muslim to eat meat is not the same as bringing flowers to a gay wedding because the eating of the meat is the sin just as the marriage is the sin. You're not forcing the florist to get married to another man. No one is punishing anyone for their belief. They are punishing people for acting on them. Discrimination is illegal. You don't get a get out of jail free card because of religion. Forget about the florist for a minute. Think of really extreme case. Someone who doesn't want to offer their business or service to a group of people whom their religion says they shouldn't mix with. What if that service is the only gas station in a small town and that group of people are driving through that town, about to run out of gas. Should they be stranded there over someone's belief? Of course, a gay couple can find another florist. Hell, they should want to find one. What gay person would want to support a businessman who thinks they are a sinner? But our laws have to be applied to all or else someday someone who really needs a service won't get one because of "religious freedom."