Bisexuality doesn't have to be an exact 50/50 split - this confuses a lot of people. It's important to recognise that sexuality is insanely fluid. While gender(but predominantly sex in the biological form)-based attraction seems to be a biological state, it can be influenced to an extent by context or external factors.
Social pressure can, without a doubt, influence an individual to broaden or restrict their sexual horizons. An individual can associate their same-sex attraction with negative feelings, while relating opposite-sex attraction to positive feelings of some sort.
The gay friend mentioned may have any number of feelings in his head that he associates with same/opposite sex attraction. This is being expressed in his public appeal for recognition as opposite-sex attracted, regardless to what extent.
There are just a plethora of possibilities for people's sexual identities.