United Airlines is investigating a complaint from a passenger that he and his partner were called a gay slur by an agent at Denver International Airport."We were completely shocked. We were shaking, Billy Canu told 7NEWS on Sunday.Canu said he and his partner were waiting for their flight home Saturday evening at DIA when they got confused about access to Uniteds Gold Lounge on Concourse B. Canu said the incident happened when the two men approached nearby United gate agents.Canu said the agents gave the couple a very condescending, sort of rude answer. They were trying to figure out who was going to help us.When Canu and his partner complained about the handling of their question, a manager approached them and escorted them away from the area. Canu claimed the manager then escalated the situation. As we were walking away he goes idiots, recounted Canu. So, my partner turned around and started walking up to him and said, 'I'm sorry. What did you say?'""He [the manager] says, What f*****s, according to Canu.Canu said the manager even threatened to kick the couple off their flight to San Diego if they continued to complain. The couple returned to their gate, where they began posting what happened on Twitter and Facebook. I started fearing the fact that we wouldnt be able to get home if we continued fighting him on it, Canu said. 7NEWS contacted United about the complaint.United does not tolerate discrimination of any kind, said airline spokeswoman Megan McCarthy. "We have received this complaint and are reviewing, and we will reach out to the customer directly."Canu and his partner said they just want an apology from the airline.No one wants to be handled in a derogatory way," he said. "We understand things can get heated, but I think their customer service couldve handled it much better."