That's just twisting another's words, which perhaps you would better understand if someone were to interpret your objections as amounting to an approval of abortion.
You asked, "So what?" Well, here is the so what. The big difference here is that slavery was outlawed long ago, whereas abortion on demand is protected and encouraged. If slavery were still legal, there would be need of standing against laws protecting and encouraging it.
As it is, those who actively oppose the slavery of the past, already deemed wrong and outlawed, are pushing and practicing a particularly hideous form of murder when they should be calling for it to be outlawed. That degree of hypocrisy is outrageous, completely indefensible, and should be called out.
Given the way you are twisting the argument, why shouldn't your apparent defense of these hypocrites be interpreted to mean that you are in essence approving abortion?