Underlying premise of "Biblical Christianity": The New Testament and the birth, life, death, and resurrection of Christ are the fulfillment or completion of the Old Testament. The NT unequivocally delineates a few forbidden behaviors for Gentiles. Among those are homosexuality and sodomy.
It depends on how LGBT Christians think of themselves. For instance, a gay man might be christian first but sin according to his desires. This wouldn't be different from every other heterosexual christian, yet commits sin regularly.This isn't like asking can a theist be an atheist.
This depends on one's interpretation of the bible and of the religion's guidelines. There are no interpretations that are objectively correct, thus the statement cannot be absolute right or wrong. We refer to religion as "belief" for a reason.
You are not truly Christian even though you may call yourself one unless you uphold, follow, and meet the requirements of the Bible. Therefore to claim to be Christian and defy or be against Christianity is illogical.