I know a guy who says he's bi-sexual but is faithful to his wife. He agrees that the Bible is the word of God, but where the Bible condemns homosexuality, he insists that it is mistranslated. What do I say to that? How do I get him to believe the truth?
Your complaint is not unusual. The fact is that your friend does know that the Bible condemns homosexuality. But this leaves him in a quandary. He wants to believe that homosexuality is righteous, but the Bible plainly condemns it. Different people will settle on different methods to resolve the conflict in their minds. One method is to redefine terms so that what is being condemned appears to no longer the subject of the verse.
Paul warned Timothy about this tactic:
"Now the purpose of the commandment is love from a pure heart, from a good conscience, and from sincere faith, from which some, having strayed, have turned aside to idle talk, desiring to be teachers of the law, understanding neither what they say nor the things which they affirm. But we know that the law is good if one uses it lawfully" (I Timothy 1:5-8).
"If anyone teaches otherwise and does not consent to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which accords with godliness, he is proud, knowing nothing, but is obsessed with disputes and arguments over words, from which come envy, strife, reviling, evil suspicions, useless wranglings of men of corrupt minds and destitute of the truth, who suppose that godliness is a means of gain. From such withdraw yourself" (I Timothy 6:3-5).
"Remind them of these things, charging them before the Lord not to strive about words to no profit, to the ruin of the hearers. Be diligent to present yourself approved to God, a worker who does not need to be ashamed, rightly dividing the word of truth. But shun profane and idle babblings, for they will increase to more ungodliness. And their message will spread like cancer" (II Timothy 2:14-17).
"But avoid foolish and ignorant disputes, knowing that they generate strife" (II Timothy 2:23).
The old cliche remains: "None is so blind as the one who will not see." You cannot make such a person see the truth, but you can challenge him to prove his position. Ask him for the sources for his conclusion. Ask him to explain why he comes to a different conclusion than what is generally accepted. Did he arrive at the conclusion because of overwhelming proof or an is he grasping at anything that remotely supports his preconceived notions. You see if someone wants to believe an error, there are always people willing to tell a person exactly what they want to hear. "For the time will come when they will not endure sound doctrine, but according to their own desires, because they have itching ears, they will heap up for themselves teachers; and they will turn their ears away from the truth, and be turned aside to fables" (II Timothy 4:3-4).
The falsehoods crumble when they are compared to what the Scriptures teach. A few will realize this and leave their false position. Most, unfortunately, will stubbornly hang on to an unjustifiable position until their dying breath. In such a case, the best thing is to move on and pray that someone else will figure out a way to open their eyes.
The study Keys to Understanding discusses the various means in which people warp the truth, how to detect this in discussions, and how to point out the flaws in reasoning.