I'm sure everyone would be happy to get a female's opinion on this, so lets do this.
While I am not a fan of prostitution, I think that it should be legalized. As KV said, legalizing it would help stop the exploitation of prostitutes.
These women (or men) don't grow up telling their parents every night "I want to be a prostitute when I grow up!" They are just like you and me, wanting to be an engineer, a lawyer, a doctor, a writer for Vogue. They turn to prostitution because they come across an economic situation where they do not have money. I am not going to lie, many of them get to this position because of bad habits they have (smoking, drinking, pot, heroin, etc.), but some get there because their parents lost their job, the stock market went bad, a boyfriend screwed them over, or some other reason.
As for the STD thing, I'm not so sure that legalizing prostitution would do that much in terms of halting the spread of STDs, though it probably would help. While I hate myself for saying this, the only 100% sure fire way to stop STDs is to somehow force everyone with an STD to not have sex for the rest of their life.
I could go on and on on this topic, but I have class soon, and need to study. So I shall leave you with these few thoughts.