I think that someone 'struggling' with homosexuality can be a Christian, certainly. We all struggle with sin. But someone who does not try to get rid of his/her sin but willingly chooses to sin is in trouble. What the extent of that 'trouble' is I dont know.
One could make the case that we all 'willing choose' sin when we sin, and that would be true, does that mean we lose our salvation each time we sin? No.
I think ity is difficult to judge without being a fair judge of which no man really is, but if I were to go out on a limb I would say that a lot of it depends upon how the individual views the lifestyle.
If a gay person doesn't admit that what he/she is doing is sin, then they are in danger of hell.
If they know what they are doing is sin,and they continue therein the line becomes very thin because we indeed all know that what we are doing is sin when we are doing it.
And finally if they are struggling with it, trying to clean up their lives from it, I can't say that they are non-Christian because all Chrsitians are struggling with sins of one magnitude or another.