Thats a hard 1,i dont agree with it either,you have to follow your beliefs and do whats best for you,its not your fault and,they shouldnt put you in that situation,ive left my bi and gay friends alone because of my beliefs,you cant help them or change them so,you shouldnt have to be subjected to that,good luck...
Just continue being a good friend. You do not need to agree with their choice but try to support it. It's not like they are going to try and make you gay too. They need friends right now especially if they chose to confide in you.
There is nothing wrong with that. That is his life,he can do what he wants.. if he wants to be bi then he can be atleast he is being his self. If it really bothers you that much then start hanging out with him less then you usually do then.
It must be nice to be so perfect you can judge others regarding their life styles. I mean, even more perfect than Jesus? When the woman taken in adultery was brought to him, he said, let he who is without sin throw the first stone, I guess this means you are without sin. Why haven't you ascended yet?