The radical feminist position on prostitution is about reducing harm to women as a class. That a minority choose to willingly engage in sex work doesn't negate the structural issues at play here. Most women in prostitution are from marginalized backgrounds and many are trafficked. Where is their freedom?
Fundamentally this is about men holding physical, sexual, and economic power over women, enforced by violence or the threat of it. Treating women as a product to be consumed and profited off, rather than co-equal individuals. What is so bad about wanting an end to this?