Slavery was actually already disallowed by the text and (arguably intent) of the constitution, but they passed amendments for it anyways just for good measure. Pointing out the slavery issue and its genesis hurts your abortion argument. The 13-15th amendments is how it's supposed to work.
Now take the gay marriage issue, I am glad that gay people can marry, but I disagree with it coming from SCOTUS.
I would think that the terror of a Trump nominated majority on the court would sober you to the idea of constitutionalism. Sadly, it hasn't.