The main issue is that the Chad Allen, who is a well known gay activist has been hired to play the lead role. And the controversy is not as simple as saying "does this mean we can't work with people who are in sin?"
It's going to take some time to read up on this issue. But, I suggest you to check out the below links. And be familiarized before commenting on this issue.
I have some opinion about this matter also. But, I would like to focus on the issues that is presented by the link I am putting here.
Pyromaniacs blog article about End of the Spear controversy
Sharper Iron's blog entry about the issue
Daniel Phillips' blog entry about the issue"
Here is an excellent points Daniel raises regarding the potential outcome of this whole ordeal...
1. If we go and support the movie, the pro-homosexual-agenda-driven MSM will report this as our "reluctantly growing" in our acceptance of homosexuality. It will be used to marginalize the concerns we express about this destructive sin, and decaffeinate our call to repentance and regeneration. Particularly (if reported accurately), Steve Saint's gushy acceptance and embrace of Allen will be messaged as acceptance of his homosexuality.
2. If we protest and stay away, we'll be shown up as knuckle-dragging haters, and once again Steve Saint's own embrace of Allen will be used against us. Why can't we be more enlightened, like Steve? After all, it's him and his dear dad that Allen is portraying. If it's okay with Steve, and Steve embraced Chad just as he is (as reported), why don't we? Our protests will be used to highlight the film. And here's the really nasty, biting irony: the "martyr" will be seen as Chad Allen... not Nate Saint.
3. If we do not support the movie, a message taken will be, "See? Christians don't even support Christian productions. Better get back to putting out filth."
4. If we do support it, expect to see Chad Allen preaching all over the place. Hint: he won't be preaching the Christ who Nate Saint died proclaiming.