"Why is it logically inconsistent for atheists to condemn slavery?"
I say that, Sabrina, for several reasons. First, because an atheist believes (or should believe) there is no objective morality. So although they might object to something personally, they should not expect others to share their view - specially if they were raised in another culture or at another time in history.
Second, when they are object to slavery, they are saying, "You SHOULDN'T do that!" But you can't get an "ought" from an "is" in an atheist's universe. Things just are the way they are. That's why Dawkins admits good and evil do not exist - but then proceeds to decry the evils of religion. If he would even stop to listen to himself for a minute he would realize how full of poop he is.
Finally, most atheist admit believe we do not have free will. Thus, those slave keepers cannot do anything other than what their selfish genes drive them to do. So it's illogical to criticize someone for doing something when they are helpless to do otherwise. If someone bumps into you at the shopping mall and spills your latte, you might be cross with them until you discover they were pushed into you. Then it becomes silly to be angry with them.
We also don't blame the lion for killing the gazelle and we don't call it murder. He's just doing what lions do. Animals have no free will. They are driven by instinct. And in an atheist's universe, if keep a slave, commit rape or shoot up a school who are we to criticize. Such is the hell that is atheism. It is a worldview which cannot be lived consistently. That is true of virtually very other worldview but Christianity. And that is why I think it's true. (Hindus believe reality is an illusion but they still look both ways before crossing the street.)
"Best do you agree with gay marriage?"
I've recently had a change of heart on this issue - primarily after watching a debate on YouTube. (See below.) Yes, I support gay marriage. I was won over because the gay man, quite frankly, had better arguments. Interestingly enough, the Christian opponent of gay marriage had toured with Hitchens, and in my opinion, blew him away in debates (which is not small feat). Douglas Wilson has a keen mind so I went into this debate, expecting him to win. I came out a supporter of gay marriage. This was only about two months ago.