Manufacturing doesn't equal endorsement.
If they will refuse to take requests in that manner, they need to look into a different line of work.
Agreed it doesn't necessarily... however it could be seen as providing material support... more importantly refusal to create/produce can easily be seen as not endorsing/opposing. That's the main point - they oppose the idea (not the customer) they refuse to create something supporting/endorsing that idea.
This is no different to a printer refusing a flier or a newspaper/other company refusing advertising.
The problem is that the idea relates to a particular group - it's not a fundamental part of that group's identity but it's something a majority of that group supports. That's where opposing that idea is being seen as discriminating against that group. While you could argue that the regusal of NI to marry gays is itself discriminatory(I'm pretty sure it is) I don't believe that refusal to support/decision to oppose the campaign is. That's simply a freedom of speech/freedom of expression issue.