> We shouldn't do A, because even though A is okay, it will likely lead to B, then C, then D, which is definitely bad.
I don't think this is a valid form of reasoning by itself. The onus is on you to show that the A > B > C > D causality chain is inevitable. Simply mentioning the phrase "slippery slope" doesn't constitute such an argument and can be dismissed without thought.
Sounds like it could be a slippery slope. I'm thinking there's a window of moral relativism to explore here.