Let's start with the easy parts. There is no Biblical basis or any teaching in Christianity which would make it acceptable to hate, harrass or do harm to someone who is gay. Anyone who suggests that Jesus hates gays or that we should hate or do harm to homosexuals is flat out wrong. This viewpoint is evil and goes against everything that Christianity is about.
There has been some media attention about this lately, so let's clear the air. A man by the name of Fred Phelps and some of his followers have been advocating that "Jesus Hates Fags." They have been attending funerals of homosexuals and holding up vulgar and offensive signs. Every single Christian on earth should find this behavior offensive and outrageous. The truth about Mr. Phelps is that his game is all about being offensive. It has been alleged that their goal is to be so offensive that people attack them so that they can sue the attacker for damages. If that's true, then it's all about money, and it has nothing to do with Christ and his teachings.
Now, to the hard part. Is homosexuality a sin? Jesus did not speak to this issue, but the apostle Paul did in his letters. Perhaps the clearest and most important passage is from Romans 1, 26-27:
Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion.
Biblical interpretation is not an exact science, but the simplest reading of this verse is that sexual relations with the same gender is a "perversion" and the penalty Paul speaks of, as with all sin, is death.
There are many arguments that can lead to another conclusion, such as to say that Paul was wrong, Paul didn't know what he was talking about, Paul was talking about something else, and so on. However, it is my opinion that the Bible is the inspired word of God and cannot be wrong. Furthermore, the Bible is never deliberately obscure and so it is a dangerous practice to twist and change passages to our advantage when we don't like what they say or think we know better what the author really meant. Your opinion of the Bible may permit you to dismiss Paul for one reason or another and if so, we will have to agree to disagree.
If homosexuality is a sin then the bottom line is that it is not God's will for you to engage in that behavior. If you are gay, then it is God's desire that you find a way through prayer, healing or counseling to change your sexual orientation or remain celibate.
If you think that sounds incredibly unfair, then you will get no argument from me. I cannot pretend to imagine how horrible it would be to have a sexual desire for men, yet to know I cannot act on it. For whatever reason, God occasionally permits us to have a desire for something which it is not lawful to have. Some unfortunate souls will battle addictions all of their lives. Some will fight with pornography, alcohol or drugs. Others are born with illness or disabilities. What I do know is that if God has chosen to give someone this challenge, then it is right in His eyes and we must accede to his wishes, no matter how unfair it seems. We will know in the end why God challenges some in this way.
What to do with a Christian who insists on maintaining a gay lifestyle? Love them, support them and pray for them. Let them know that you will help them in anyway you can. Whatever you do, do not turn your back on them. If they will not turn away from that sin, let them know that you disapprove of their choice and continue to pray for them, but let them understand you are still there for them and will love them no matter what. You may not wish to do anything thats shows support such as attending a gay wedding, but I think it would be a huge mistake to cut them out of your life. Nothing good can come of that.
Are homosexuals who have asked Jesus to save them going to hell? Certainly not. The saving power of Jesus covers all sin. It grieves Jesus that anyone should choose to sin after giving their life to Him, but He knows we are weak and He will forgive us and save us.
It can be a lifelong battle to overcome unlawful desire, addiction or temptation. With prayer and the help of the Holy Spirit, we can overcome anything. If our faith is weak and we fail, at least we know that if our faith is in Christ, we will be saved no matter what we have done.