How is it that you "often hear" terrible stories like that? Do you hang out with Hollywood gossip columnists?
Well those seedy rumours keep cropping up on talk forums and blogs every so often, but I don't think once being a sex worker or/and pornographic performer is detrimental to anybody who got him/herself out that sort of dead end and moved onto better things (I heard that Hugh Jackman was supposedly in gay porn, but I don't think it has any real bearing on his present day career).
I've liked Michelle Pfeiffer since
Scarface in the 1980's, and I have personally never read or heard that she was a prostitute, escort or whatever. When would she have done this? She was already getting work on network TV by age 20 (she was born in April 1958 and her TV show "Delta House" aired briefly starting in January 1979). During what years would she have become so disillusioned and desperate?