Although I am gay I do believe bisexuals exist
and I did think Duncan James was bisexual as a) that was what he said about himself and and b) he seemed to like women at some point. However lately his statements about himself have suggested that he is gay. Perhaps there was a point where he was bi and he was simply being honest about that rather than some planned public move from straight to gay where bisexual was a stepping stone. I remember George Michael saying there was a point where he thought he was bi and had relationships with men and with women and then he realised he simply preferred men and that was that.
The Jessie J one is interesting. She seems very keen to be clear that she is not lesbian but bisexual. I appreciate she might want to set the record "straight" (arf) and her bisexual identity might be very important to her, but the speed with which she was tweeting that she wasn't lesbian but bisexual when someone tried to out her as 100% gay was kind of intriguing. The rumours I hear on the scene in London (I am not saying these are true) is that she has a long-term gf and has not dated any men since she was about 17 or 18. Of course if these were true, she could simply be a bisexual woman who is committed to her girlfriend, and if Jessie J says she is bi then she is bi.