I dont agree with this. I dont see any reason to accept that for Jesus' teaching to be a good source of morality, that he must accept and comdemn every possible action as either moral or immoral. Even if he had condemned slavery (like he condemned many other things) there would be other things that he did not speak against and we could play this game until the end of time. Just because he didnt explicitly speak against X it does not mean that we cant conclude what he would have thought about X based on other moral norms at we can attribute to him. Now, this isnt even a problem unless you believe that 1) the bible is the literal word of god (minority of christians) and 2) that it is the complete word of god with no possible omissions (an even bigger minority of chistians).