When I accepted the job I was not told I might be asked to deliver gay magazines. When I was asked to deliver them I said no - they were then free to sack me; they decided to keep me on.
Bullshit. You were asked to deliver the magazines that the shop's customers ordered, not pick and choose. It may not have occurred to you when you took the job that that might include gay magazines, but when you found out that it would, your responsible courses of action consisted of:
1. suck it up and do your job
2. offer your resignation