And many were raped. Repeatedly. In a culture where this was thought of as normal. How can a relationship be consensual and loving when one partner literally has the right to order the other, to harm the other in any way, to literally kill the other? If there were so many consensual and secret relationships, why did white slave owners talk so openly about rape of black women (and white women actually, read William Byrd Carter II's diary, it's a shocking horrible thing), and speak so strongly against any sort of liason between white women and black men? Black slaves lived in a world where killing your rapist would get you found guilty [1]. Surely there were some loving relationships (people are diverse), but just as surely the majority were not.
And what were slave-owners trying to protect slaves from? If slaves had it so good under slavery, why did thousands and thousands attempt to flee at great risk to their own life? Why did they overwhelmingly vote for the Republican Party after reconstruction, again at great cost to their own lives? Why did so many who did flee write about the evils of slavery, and campaign against it, and no free black wrote for it? All slaves were owned to extract economic value of them, they were huge capital investments.
[1]http://law2.umkc.edu/faculty/projects/ftrials/celia/celiaacc...