Why should some people get to tell other people
what they can or can't do with their own bodies?
The reasons to oppose trafficking, under age prostitution and violence against prostitutes are obvious. And it's obvious why home owners wouldn't want street walkers outside their front doors. Let's set those aside and consider indoor prostitution without slavery or violence.
1. In many places, to collect unemployment benefits you have to be searching for work, and if you're offered work you must accept it. Would you like to see your daughter or mother denied benefits because she turned down a job offer from a brothel?
2. There are a bunch of things we can't consent to. Even if I sign a contract agreeing to be sold into slavery, or agreeing a loan shark can break my legs, or agreeing to work for less than the minimum wage, that part of the contract is invalid. There's precedent for us making it impossible for people to consent to things.
3. If there's someone I wouldn't otherwise have sex with, but I have sex with them to avoid being stabbed, that's obviously rape. If I do the same thing to avoid being starved, is that so different? Oh, admittedly it's more of a grey area, but surely punters could hold themselves to higher standards and avoid things that are even a grey area?
4. Trafficking and violence are giant problems. About 90% of prostitution is one or the other, and legalising prostitution will increase demand leading to more of both. Isn't it better to sacrifice that small fraction of nonviolent prostitution to help clamp down on the much larger amount of violent prostitution? Oh, sure, the 90%/10% ratio might be a bit off, but slavery and violence are really serious, their impact should be given greater weight.
Not saying I agree with all of them, but I can understand why a politician might decide they have other changes to the law they'd like to see first.