A person who happens to think they're gay, while actaully being straight. This person may be so conviced that they are gay, that they will do anything gay people do, untill some time down the road they find out that being gay isnt for them and switch back, therefore they are only gay for a certain amount of time, like a bad fashion. note that being poser gay may mean a sudden change from natural gay.
person 1 : "look at that guy hanging out with the gays"
person 2 : "that guy so poser gay"
person 3: "yea, he's so gonna be straight by next week"