So would you. The idea of slavery being immoral is relatively new and just as 1000 years ago, we would not have discussed air fares, we would also not have discussed the abolition of slavery.
http://en.wikipedia.org/wiki/Abolition_of_slavery_timeline
I would have only out of ignorance. Once I had been pointed out that slaves have the same felings and wants as non-slaves, I.e. not been ignorant any more, I would not have supported slavery no matter how beneficial it is to the economy and non-slaves. I currently support things like gay rights, rights for women to receive equal pay, rights of immigrants and non us citizens, etc, all things that currently still run contrary to US law, and I support them despite the law saying I shouldn't.
To go even further, I promise you I will support the rights of clones, intelligent robots, intelligent human-animal hybrids, human cyborgs, and intelligent aliens, even if the laws passed said they should have no rights because they are not "human." I fully suspect those on your side will be the ones passing those laws, and denying those people rights, just as they were denying the rights of blacks, irish, jews, women, gays, transgendereds, non-christians/muslims, atheists, immigrants, etc. etc. etc.