I'm upset one of my guy friends came out as gay after dating another friend of mine, a girl, for two years.
Do you have a problem with gay relationships or gay people?
I don't care about relationships, but what bothers me is they lied to me for two years about their sexuality.
I'm sorry that you feel lied to by your friend. As people grow, they may learn more about themselves and their sexuality, as it sounds like your friend did. I think you should be supportive instead of offended.
I'm going to say something really offensive to my friend.
While this is new for you to process, under no circumstance should you be wrong and homophobic towards your friend. Not only is it awful and wrong, but your words and feelings could hurt him for a very long time well into the future. He's still your friend and you should reevaluate your perspective.