Michael Bird has got an excellent writeup on the issue of whether or not Christian businesses should provide their services when those services are being used in conjuction with something that conflicts with the Christian faith. It is well worth a read.
I tend to agree with Dr. Bird in that providing our goods and services for customers that have conflicting views with our own is precisely what it means to be in the world but not of it. In addition to the Scripture references that Dr. Bird makes, we have Jesus advising us that if we have the opportunity, to make friends with those who have worldy wealth (Luk 16:9). There was no requirement that they be Christians.
I think seeing this particular issue as being forced to do something contrary to one’s faith is a perspectival argument. There are other ways of viewing the situation that seem to fall in line with how God advises us to behave. No one is asking the businesses to agree with homosexual wedding ceremonies. Walking a mile with someone doesn’t mean that we agree with their theological views. Pastor Greg Boyd has said that the only judgment we should make concerning a person is to agree with God that he or she has “unsurpassable worth” (Boyd, Greg A. Repenting of Religion. Grand Rapids: Baker Books, 2004). I think Boyd is helpful here.
If we attempt at making discernments regarding the receiving end of any part of our work, I think we need to remember that while we yet sinners, Christ died for us too.