They're also really, really against getting caught having anything to do with anything illegal, and there's a lot of illegal stuff associated with porn. It may be only a small percentage, but the last thing they want is to be sued or criminally charged.
So, they are very, very wary of anything to do with porn. I'm sure they'd be happy to have their cut of that very lucrative industry, but they'd rather forego it than having their entire business destroyed over it.