Simple. Just don't bring up the topic.
I have lots of 'friends' and people that I care about, that are LGBT, atheist, etc. Generally, they're respectful and stay away from those controversial topics; from my experience, it's only the absolute horrible people that wave the topic around and argue illogically about their side/cause. (hint hint)
Also, this specific issue that you've brought up, (you being orthodox, and they being atheist), this is a huge social issue that you alone won't win against. Christianity is a dying religion here in the United States. My personal suggestion would be that you ignore the topic, and treat them like your 'normal' Christian friends. Also, I must add that I feel rather suspicious of how you've typed this, given that the LGBT and Atheists are humans, and deserve as much respect and love as us Christians; you seem to think that you must play missionary, or preach to them about Christianity, which leads me to my final two statements;
Don't play the missionary. It'll only cause bitterness and arguments; you won't win that fight.
Unless your issue is about talking Christianity to them, and you trying to play missionary all along, and not how to get along with these people... in which case, do read the two statements above. ^-^