Hi Dom,
I'd like to share some thoughts, although they might be gloomy, and although I might not be an expert in psychology:
Parents, especially mothers, are prone to ignore reality when it comes to their children. They want them to be happy, and to gain happiness through their happiness. Happiness, in their eyes, implies being married and having children. Deep inside they don't put up with a child not living up to this pattern; they clutch at every straw promising to prevent it from going on with their "deviant" behavior. In my eyes, that Christian propaganda you describe thrives on this disappointment. Interestingly enough, it seems that parents often react in a similar way when their daughter tells them that she wants to become a nun. I therefore suspect that it's not primarily the Christian mindset causing mothers to be adamant on changing their child's homosexuality, but some (genetically coded?) notion of desired behavior. Sticking to that pattern is supposed to provide happiness. Refusing to do so is perceived as challenging the family or society, which is deemed to be unacceptable. Mothers may still love their homosexual children, though, even if they don't accept them, and feel compelled to carry on with "helping" them.
But surely this is just one aspect of the issue, and I hope that you will succeed in putting your mother right. Remember that she is acting in good faith.