The former N-Dubz star is on trial at London's Southwark Crown Court for being concerned with the supply of cocaine.
On Thursday while undercover reporter Mazher Mahmood, dubbed the "fake sheikh", was giving evidence, the court was played a video in which Mahmood is heard asking Contostavlos' PA Gareth Varey if Mr Cowell was gay.
Mr Varey replied: "Yes", prompting Mr Mahmood to ask: "Have you slept with him?"
Mr Varey replied: "No" and when he was asked why, he answered: "I will when I make my move."
Mr Mahmood then asked: "How do you know he's gay? Mr Varey replied: "I know people who have."
A spokesman for Mr Cowell said: "We are bound by contempt of court rules so cannot respond to these claims in the manner we wish to.
"We are speaking to the Attorney General's office - and we intend to make a statement in court as soon as possible."
Contostavlos denies being concerned in brokering the drug deal.