Have a source for that statistic? Seems like a somewhat broad generalization.
Just my (admittedly limited) personal experience, but seriously. I can't remember a single female psych patient who denied a history of sexual abuse, and it seems like half the SA patients that come in just don't want to live with their PTSD anymore. Don't know what the cause-effect relationship is here (it's likely two-way), and it's certainly possible that some of it is factitious/delusional, but I don't think anyone on here would deny that it's rampant.
EDIT: Okay, a quick Google search shows:
51% of female state psychiatric hospital patients were found to have a history of childhood or adolescent sexual abuse (1988, Hosp Community Psychiatry; n = 105)
http://www.ncbi.nlm.nih.gov/pubmed/3356438
Meta-analysis of 37 case-control and cohort studies showed that sexual abuse history was related to an increased risk of anxiety disorders (OR 3.09), depression (OR 2.66), eating disorders (OR 2.72), PTSD (OR 2.34), sleep disorders (OR 16.17) (!!!), and suicide attempts (OR 4.14). There was no statistically significant increase in risk of schizophrenia or somatoform disorders. They didn't find good data on bipolar disorder or OCD. (2010, Mayo Clinic Proceedings; n = 3M+)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2894717/
We're dealing more with what I'd call "para-psychiatric" disorders, but the same people did a meta-analysis of 23 case-control and cohort studies showed that sexual abuse history was related to an increased risk of IBS (OR 2.34), nonspecific chronic pain (OR 2.20), psychogenic seizures (OR 2.96), and chronic pelvic pain (OR 2.73), with no association found with fibromyalgia, obesity, or headaches, with insufficient data on syncope. (2009, Mayo Clinic Proceedings; n = 4640)
http://www.ncbi.nlm.nih.gov/pubmed/19654389/
The data on schizophrenia are interesting, in that there doesn't seem to be a consistent link between childhood sexual abuse and schizophrenia, but I guess you might expect that given that schizophrenia is more 'biochemical' while many of the others listed are more 'psychological' (and to any Psych people out there, please don't flame me for the oversimplification!). However, while an association between sexual abuse and the later development of schizophrenia is debatable, there does seem to be a clear association between schizophrenic patients who
then become victims of sexual abuse. It's a broad topic, but I'll link to
this study and
this article as examples.
Basically, no, maybe it's not 100%, but given the problems with sexual abuse studies (i.e. underreporting), it's probably safe to assume that a female psych patient should be approached with extreme sensitivity and caution when it comes to genital, rectal and breast exams.