So the qualification would be whether a tiny subset of the population of what, for all intents and purposes, was a different country (NY vs. VA) had raised an objection to a practice that had been continually used for thousands of years?
Look who's the moral relativist now. This can't possibly be how you judge such an idea. Rape, murder, confinement, and shunning have been used for thousands of years throughout the world to keep women down. Can men in rural Pakistan say that honor killings are fine by the standards of their time because only a subset of a population of another country feels they aren't? Does that suddenly change when a subset swings from 49% to 51%?
Washington lacked the foresight, intelligence, and compassion of his abolitionist peers. It's not like there was no way for him to have known better. He just chose not to do the right thing. I don't see what's so hard about this.