It is never wrong to take a stand for Jesus Christ -- which I believe you were doing in this situation. If you had followed his instructions, you wouldn't have been just taking their money (as clerks must do), but you would have been actively urging people to do something wrong: fill their hearts and minds with impurity and lust.
Remember too that those you worked with undoubtedly knew you claimed to be a Christian. What impression would it have made on them if you had compromised and urged people to do something you know is wrong in God's eyes? Either they would have decided your faith wasn't real after all, or they would have concluded God doesn't care what we believe or how we live. Either way, your witness for Christ would have been blunted.
Never forget Jesus' words to His disciples: "You are the light of the world. ... Let your light shine before men, that they may see your good deeds and praise your Father in heaven" (Matthew 5:14, 16). God will honor us when we follow Him.
As a footnote, pornography has become a major problem in our society today, not just on the printed page but also through the Internet. It is one more sign of our spiritual emptiness, and our need to return to God. May the prophet's prayer become ours: "Revive thy work in the midst of the years" (Habakkuk 3:2, KJV).