Natural law and what's called "divine positive law" (what God has positively told us in Scripture) can help immeasurably. 

Natural law and divine positive law are like each others' guardrails; they enlighten the way for us; neither can contradict each other.

And of course other positive law (e.g, civil law, canon law) can't violate these other higher forms of law (natural law, the laws of nature, and divine positive law).

We have to look through the lenses of these higher laws in order to adjudicate the "positive" law that humans come up with.

Canon law can never violate natural law in other words.  

The Church can't come out tomorrow and say "same sex marriage is acceptable".   The guardrails are implanted. 

And we don't bend or dismiss this or that scripture to suit our fancies.