...it is illegal based on the explicit wording of the Constitution, and, hence, definitionally unconstitutional.

In contrast, the Constitution makes no mention of contraception one way or the other. It's true that one could make an argument for it based on privacy clauses, penumbras, etc., like those used in the Roe decision to legalize abortion, but that's a steeper hill to climb, whereas the legality of slavery is not in question. They are not the same thing. And, again, I'm totally in favor of contraception -- this is just a debate about where the lines are drawn between the Constitution, federal law and state law.