It would be good to clarify whether we’re discussing prevalence or incidence of SA. Prevalence is the cumulative number of people who have experienced rape/SA and incidence is the number of rapes/SAs that occur in an area in a given year. So independent of the legality of prostitution, you could have a very high prevalence of SA (on the thought that people who are involved in prostitution and sex work have a higher than average rate of victimization histories) and incidence that is about average for anywhere else.
Second, it is impossible to know the “true” prevalence or incidence of rape/SA in an area—are you measuring the prevalence in the population that lives there, regardless of when or where the assault occurred, or are you measuring the incidence in that area? The two most common ways of measuring both are either through aggregating reports to official agencies (police, hospitals, rape crisis centers, DCF, etc.), which is subject to serious underreporting; or, through self-report data in surveys, which are subject to response bias and are hard to compare across surveys because of differences in methodology (some surveys only ask people about completed rapes/SA where others ask about completed or attempted; some only ask about experiences after age 12, or within the past year; some ask people if they have ever experienced “rape” or “sexual assault” whereas others describe actions that would meet the definition of rape/SA and ask if people have experienced that).
OK. So. Setting aside those issues, does legalizing prostitution in an area decrease the incidence of sexual assault? This is a complicated question, for a couple of reasons.
First, I think there is inherent in that question the assumption that sexual violence occurs because perpetrators have a dearth of access to consensual sex, which presumably would be alleviated if they could just pay for it over the counter. We know this not to be the case, because research by a number of academics on both incarcerated and undetected perpetrators finds that they self-report having access to consensual sex at the time they were committing assaults.
Second, it is possible to make the argument that prostitution and sex work actually result in more sexual violence, because whether legal or illegal, prostitutes and sex workers are not seen as credible victims or witnesses by the criminal justice system or general public. There was a 2002 study of street-level sex workers (which is considered to be the most dangerous form of sex work, as opposed to escort services) that found that 72% of the female prostitutes surveyed related instances of severe abuse (rape, being beaten with objects or threatened with weapons, or threatened with or abandoned in remote areas) at the hands of their partners, clients, and/or pimps. Respondents also reported that they rarely reported such abuse to law enforcement (Dalla, RL 2002). An older study of 130 people working as prostitutes in San Francisco that was studying the extent of violence and PTSD in that population found that 57% reported they had been sexually assaulted as children and 49% had been physically assaulted as children. As adults in prostitution, 82% had been physically assaulted, 83% had been threatened with a weapon, 84% reported current or past homelessness, and 68% had been raped while working as prostitutes (Farley and Barkan 1998).
There is a larger feminist debate around prostitution and sex work that is similar to the one that occurs around pornography: is this an example of an industry in which (primarily) women’s valid use of their bodies for work is criminalized and undercompensated, and that with proper regulation and oversight could be a viable profession? Maybe. The flip side to that is that it commodifies sex workers and that you are never an autonomous freelancer, but always already a victim of a culture that sees women only for their sexuality and as vehicles for men’s sexuality, so legalizing prostitution just puts the imprimatur of the state on that situation.
My personal position is that—legal or not—there is much that can and should be done to improve the safety of prostitutes and sex workers, and what we do around issues of homelessness, substance use, and mental illness has a lot to do with who ends up as a prostitute (i.e. because of “survival sex” traded for necessities), and thus at greater risk for sexual violence victimization. From a prevention lens, I don’t know that the illegality of prostitution does much more than scratch the surface of rape culture.