I think we always have to go with our conscience. As I see the matter, there's a difference between driving a truck or copying brochures and counseling a gay couple or performing the service.
Further, since a gay/lesbian union isn't a biblical marriage, I personally would not want to be viewed as endorsing it. Yet Christ-followers should always strive to be friendly, respectful, and non-discriminatory -- and sometimes when I strive for these qualities I may be perceived by some persons as going too far, of compromising. Even Jesus was criticized for the company he kept, as well as for his eating and drinking habits. Even if we conduct ourselves with holiness and probity, we will be criticized.
Of course it's wrong for anybody (for example, in the government, social services, or in activism) -- an ACLU-type bully -- to try to force anyone to act without regard to their convictions and conscience. We stand before God, the judge of us all (Rom 14:1-12).