John Constantine is a British working-class warlock, occult detective, and con man. He is cynical, crude, and a chain smoker, but also has a profound desire to do good in the world. Although he is a warlock, he prefers to use his cunning and rarely resorts to spells. 

His bisexuality was made clear in 1992 when Constantine talks about the fact that he's had girlfriends and boyfriends. 

Cartoon frame from the comics where John talks to someone and mentions his bisexuality.
Image/DC Comics

He goes on to have relationships with men and women in both the Hellblazer and Constantine series and recently concluded the comic series John Constantine: Hellblazer (2020–2021). 

In 2005, Keanu Reeves portrayed a version of Constantine in a movie of the same name. 

Currently, the Welsh actor Matt Ryan is portraying Constantine on the CW shows Arrow and Legends of Tomorrow. He also voices the character in Justice League Dark and Constantine: City of Demons. The show has had no problem showing off Constantine's flirtatious nature.