Back when slavery was still being practiced in this hemisphere one of the marks of the slave was their inability to enjoy the benefits of the civil marriage contract. The slaveholders resisted any notion of them having access to that, in the knowledge that many of the most important benefits of liberty are associated with it. Inheritance rights. Ownership of real property. Parental rights. And a whole lot more.
Those who won’t fiercely defend the institution of marriage in this country, at every level and on every front, are, whether they know it or not, helping in the drive to reduce this free people to the position of slaves.
What on Earth are you talking about? Obviously, two slaves getting married would not change the fact that they owned nothing, therefore could bequeath nothing, and could take no action (regarding their children or anything else) without the master's approval.
If anybody cared about slaves getting married, it was simply a matter of masters not wanting the annoyance of keeping track of them as a "package deal".