Way overplayed, this thing about "period" morals. It's an attempt to equate period morals with period acceptance levels, which do change over time and between various cultures.*
The only aspect being considered here however is what was constitutionally legal.
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* Humans born into any age or culture have an innate moral compass. It can be supplanted with enough societal pressure, the popular acceptance level. For instance, killing one's child is innately immoral but in certain societies becomes acceptable as a religious sacrifice. For another instance, Americans North and South accepted slavery despite their innate moral compass directing them that it was wrong. Acceptance leads to permission, permission sometimes to horror.
If you are arguing for an innate moral compass as you suggest then you're talking with the wrong person. I think that's make believe. What one group or person considers extremely moral another thinks completely immoral. More of human history is full of concepts of defined superiority and inferiority between other humans and living being than the concept of universal equality. In fact universal quality is a fandangled new idea in regards to human history. Much of human history in fact believed that tiers or castes of people existed and it was in fact moral to give the different groups different rights and immoral to do otherwise.
You say what they believed was moral wasn't in fact morals, yet many of them actually used the exact term moral and the exact concepts. I can either believe the people of the time or believe you about what they actually believed. You can guess which I choose. I'm sorry I don't buy your invented terms compared to the actual contemporary terms uses in their actual contexts.
As to only talking about what is legal or constitutional, there are countless threads considering many other aspects of this time period, including many great discussions and some great threads (especially by @ForeverFree) about the groups in the North that believed slavery was in fact immoral at this time and was a motivator in their actions.
With any proper study of history you will see a detachment from modern morals however. The exception being an article, study, comments, discussion specifically centered on considering the past within the context of the modern etc.