Well, in many cases, these "Christians" fail to provide why not doing business with certain people is protected under the First Amendment. A large part in winning that argument is actually demonstrating that it is actually against your religion to provide product and/or services. For Christians, it is difficult to prove that because no where in the Bible is that claim made, in this case, against gays. Now, there are verses that seem to make it a sin and to condemn the sin...but the Bible does not say that you will suffer punishment for dealing with people who commit the sin. It says you will suffer if YOU commit the sin. There are also many verses that talk about everyday dealings with sinners which basically says that such dealings will not harm you, but may provide you an opportunity to talk about God.
As such, and for other, more secular reasons,many judges rule that the person providing product and/or services must provide them, no matter who or what the recipient is.