A Christian baker should not have to bake a wedding cake for a gay "marriage".
Correct. Under the law, he doesn't have to. He can sell doughnuts, cupcakes, danishes, etc. instead. But if he offers wedding cakes to the public, then he has to offer them to the entire public.
Likewise, should a muslim photographer be forced to photograph it?
No, a Muslim photographer should not be forced to photograph a gay wedding, because he always has the option of getting out of the wedding photography business. He can photograph dogs or nature scenes instead.
Freedom of association. It's in the Constitution.
The Constitution also gives the government the power to regulate commerce. The courts have ruled that absolute freedom of association does not apply to commercial services offered to the pubic. You are free to disagree, but unless you are appointed to the Supreme Court, your opinion doesn't matter much.