So off hand, no, I don't know of any concerted effort to cease identifying slaves by their sex. It is an unfortunate comparison to make, but there is nothing inherently humanizing in admitting the sex of a slave, any more than it is humanizing to refer to horses as stallions and mares. That isn't something that makes humans unique from animals. Now, that being said, while I don't know of any concerted effort which revolved around pronoun choice, it is fairly easy enough to say that targeting of slaves sexuality was a fairly common thing, just not in that way (and hardly restricted to slavery. The use of "boy" to refer to black men is a long enduring trope). Whites felt threatened by ideas of black masculinity, and worked to suppress it through various means, both active and passive. This was intricately tied into ideas of honor, essentially a synonym for 'masculine identity'. White men had honor, and black men did not. Emasculation was a very explicit component of slavery, not literal of course (at least in the US), but denial of masculine honor. Although focused on dueling, this previous answer I wrote goes into this nicely, I think illustrating first how white slaveowners excercied control over their male slaves denying them honor, and also how the enslaved men where aware of this, and attempted to find ways to assert their identities as men. Hope that helps, although certainly can do my best if you have any other follow-ups.