Here is a better analogy.
Let’s say you are an artist. You have a gallery in which you sell existing paintings but you also do custom portraits. If a gay couple wants to buy one of your existing paintings and you refuse because they are gay, then you are discriminating against them on the basis of their homosexuality. If a gay couple wants to commission you to paint a portrait of them kissing (or nude or having sex) and you refuse, are you discriminating against them on the basis of their homosexuality?
Yes, that is an extreme scenario, but entirely analogous. Sweet Cakes did not refuse to sell a lesbian couple any products off their shelves. In fact, the lesbian couple were existing customers that the bakery had sold products to before. They refused, however, to create a wedding cake for them and decorate it to celebrate a gay wedding that they found offensive to their religious beliefs.
If you do portraits of heterosexuals kissing (or nude of having sex) then yes you are. If you have never done such portraits then no you aren't.
They refused, however, to create a wedding cake for them and decorate it to celebrate a gay wedding that they found offensive to their religious beliefs.
And I'm not arguing that they should have suffered the punishment they received. The problem with cases like this is that one side can have their rights respected only when the other side has their rights denied. Legally the customer had the right not to be discriminated against based on their sexual orientation. Constitutionally the Klein's have the right to practice their faith as their conscience dictates. Rather than choose sides the government should never have been involved.