The question in the OP asks:
Can gays or bisexuals be Christian?
Is this really asking, can a person who is sexually attracted to both males and females be Christian?
Isn't the answer a resounding 'No'?
A Christian is a follower of Christ - One who follows his example, and teachings.
So in order to be a Christian, one must be applying the teaching of Christ in one's life.
It sometimes take time for one to do this, based on his/her/it background.
For example, a drug addict may take time to get over a bad habit, but when they work at it, in time with God's help, they can overcome the temptation to give in, and they may get to a point where their wrong desire is suppressed.
When that person reaches a reasonable point in their progress - then they can become a Christian, because they have repented and made a resolve in their hearts to do God's will.
Of course, no one changes overnight, so there must be a way to help the person to change their thinking. Learning of God's ways will help in that regard... according to the scriptures.