Ultrasound Deejays said it would not be able to provide the music for a party Tom Tsakounis was planning for his 60-year-old house-mate.
The organisation's co-owner Michael Lampiris told the Washington Post: "I just said, 'We won't be able to do it, we're a Christian organization and it would go against our faith, I'm sorry.'"
He added: "It's important for us to make a stand. We don't want to go against the law, but we also sometimes are called to do that if it goes against your faith.
"To me it would be like a synagogue having to cater to a neo-Nazi party or black DJ having to do a KKK dance."
A case has now been registered against the DJ at Montgomery County Human Rights Commission, which hears cases of alleged discrimination.
The party was due to take place in the state of Maryland which has laws banning the discrimination against LGBT people in the provision of good and services.
Mr Tsakounis said the rejection "made me feel like I got hit in the stomach. You feel like: 'C'mon, in my neighbourhood?' But: Wow, yes - in my neighbourhood."
Ultrasound Deejays has a written company policy which states: "We will not be involved in any event involving homosexual celebration or activity. We follow biblical morality."
The company also said it would not play "vulgar music, tolerate provocative dancing or be involved with strippers, fortune tellers, psychics, or magicians".