Being forced to have sex with someone because you have been sold into prostitution is not a mortal sin.  There are many mitigating factors there.  And Humane Vitae is about marital sex, not sex between a prostitute and a client.  As Pope Benedict pointed out, a prostitute who uses a condom doesn't sin by using the condom.  In fact, for a prostitute regardless of sex, it is a responsible step to use a condom.  I am not sure what the big deal is unless you think that a female prostitute being knocked up with a HIV infected baby through one of her "clients" is a good thing.  I certainly don't.  These women should be using condoms until they can extract themselves from that situation.