Speaking as a former prostitute myself (from ages 12-16, I'm 36 now) honestly I have very little idea as to why people think it shouldn't be legal. While I was in sex work, I was raped several times, but if it were government regulated that would very likely be better. I didn't report any of what happened to me, because I was ashamed and I didn't want to be charged myself for prostitution.
Since it's been twenty years since I really did sex work, and it wasn't in the US, I can't really say about the political and economical impact.
Doesn't anyone realize that the legalization of prostitution would possibly bring down the number of child prostitutes? If it were legal, more people would be prostitutes, therefore there wouldn't be as much of a "need" to kidnap people and force them into prostitution. Yeah, it's not a 100% outcome, but it's going to happen either way and if there's even a chance of it helping then it should be taken.
On STDs and the like, it's a person's own choice whether or not they want to get tested or use condoms. I personally always used condoms, except for the times when I was raped, and I never got anything. Now, I do think it should be required to disclose any STDs to the customer, but if you can normally have sex with someone with an STD then why can't you pay for it? I can't speak on pregnancy, because I'm a cis man, but it should be the same as if you're just normally having sex.
And yeah, there is the huge amount of prostitutes who are raped, but that's another thing that's going to happen whether or not it's legal. But the legalization would help them, because we could report it and not risk being charged ourselves for prostitution.
This ended up a lot longer than I meant, but whatever. It gets the point across.