They're not really "opposite" assets though. I might, the S&P is an index, not an asset. Second, the assets making up the index aren't really the opposite of BTC, just...different. BTC is best treated as a commodity (like gold), while the S&P is made of actual stocks that give you ownership of a part of the companies.
We cover a lot on the intersection of crypto and Wall Street. If you're interested, check us out or drop us a line! :)