No, they aren't bi. They've said before that they really wanted to be gay, and tried very very hard, but in the end they had to accept it wasn't going to happen.
I suppose one could consider Bowie bi, but that would be stretching it by my definition... I mean, okay, he was not-straight enough to allow him to engage in some experimentation and maybe carry through a tryst or two. But that also depends on how you'd define "tryst"...
...ack, I almost went and typed out my specific opinions on how far he's gone with whom/when, but this board so does not need that.
I shall keep my specific opinions both on Bowie's sexual experience and Mika's actual sexuality to myself.