The only ones that win when you legally allow discrimination are bigots.
Do you think a gay fashion designer should be forced to make a dress for the wife of a religious fundamentalist?
A gay wouldn't bother to be a hateful little dweeb like fundamentalists are.
Should a t-shirt printer - whether gay or not - be forced to print t-shirts saying gay people are going to hell, if they have a paying customer?
Are we getting into who makes the Westboro Bapist merch?
I could think of any printing company having issue with printing openly hateful things about anybody. What you are talking about is in line with the white shop owners in the South in the Civil Rights era refusing service to the blacks. To what autonomy do we give the owner? Where does the ownership give way to the social contract of running a business? I think at the end of the day, what you must ask is if this is actually a problem, or could it really be a problem? Is forcing blacks into certain food establishments a problem? Is letting an EMT refuse treatment to an injured gay problematic? Is lack of ability for bigots to get custom garb a problem?
Also, does the dress being made by the gay for the religious have anything to do with the cultural divide between the two, whereas the gay being forced to make a "Gays are going to Hell" t-shirt... well... it's quite upfront, no?
It's certainly not black and white, and it probably has to come down to this: Do we have reason to believe the refusal of service is purely to do with discrimination by the owner? In that sense, can't we say that the t-shirt printer would have a vesting interest beyond that individual customer in NOT producing an "I hate Gays" shirt?