Gays can buy any cake offered for sale. It is unfair for gays to force me to bake a cake that is offensive to my religion. It would be unfair to force an artist to sculpt or paint an indecent work of art.
In states/localities in which anti-discrimination laws apply to sexual orientation are in place, the question the baker has to ask himself is pretty much "would I be willing to make the exact same cake for a straight couple?". If the answer is "yes" then, failing to make the cake (or rather, supplying the cake, as you can use a third party if need be) is a violation of the law.
Scenario A:
- "Hi, I'd like to order a cake, it's for a same sex wedding".
- "Sorry, we don't make wedding cakes"
(Legal)
Scenario B:
- "Hi, I'd like to order a cake, it's for a same sex wedding".
- "Sorry, we make wedding cakes, but not for same sex weddings"
(Illegal)
Scenario C:
- "Hi, I'd like to order a cake, it's for a same sex wedding. I'd like you to write 'yay same sex marriage' all over it.
- "No. I have a religious objection to that"
(Legal)
Scenario D:
- "Hi, I'd like to order a cake, it's for a same sex wedding. I want one that looks just like this one in your brochure here."
- "No. I have a religious objection to that"
(Illegal)