If you're asking for a custom product, they can deny making that custom product.

They don't have to draw things that go against their beliefs, be that drawing black people or drawing gay people , or whatever. It's incredibly disgustingly bigoted, but that falls into their own choice.

However, they can not discriminate against the customers themselves. You can't say "you're black, so you can't get a regular cake", or "you're gay, so no regular cake". If you sell it to one customer, it must be equally available to all.

Same way I can say "no, I ain't writing "god hates fags" on the cake",but I can't refuse service to the person because they're a homophobe.