It is, but whales are pretty close to the top of that slope. If you elected not to kill any animals due to probable sentience, the short list would probably look like dolphins, elephants, and whales.

I mean ... it slopes down to vegetarianism. I like meat, but a world without meat doesn't sound like hell or anything.

Probably not what you meant, but I have kind of an aversion to the phrase "slippery slope", as I think people use it to imply that we shouldn't consider a point because we might not like all the ramifications.

If it turns out all the things I eat are writing ballads about how sad it is that they all get eaten, I'd rather know than not.

It is indeed a slippery slope, but it's much easier to have whales on the not kill side when they do things like pass the mirror test