Paying for porn with bitcoin puts all of your porn purchases on a public ledger. I don't think too many people want that.
If your significant other works for a Three Letter Agency, then maybe it's possible they could reverse the network connection logs to the exchanges, dig through their databases and prove that you were the one who owned the coin that bought that porn. Maybe. But given how little the FBI has done with the cache of bitcash they seized, I doubt it.
So admittedly, it's not as clean as a cash business... but neither is the sex industry.
Granted there is the startup cost..