Voluntary sex causes as many injuries as rape

That finding really surprised me at first:

Nature, frequency and duration of genital lesions after consensual sexual intercourse—Implications for legal proceedings - Birgitte Schmidt Astrup - 2011

The purpose of this study was to make a normative description of the nature and duration of genital lesions sustained during consensual sexual intercourse, using the three most commonly used techniques; visualisation using the naked eye, colposcopy and toluidine blue dye followed by colposcopy.
Methods

Ninety eight women were examined within 48 h of consensual sexual intercourse. Fifty of the women were examined twice again within the following 7 days of sexual abstinence after the first examination.
Results

The participants had a median age of 22.4 years and 88% were nulliparous. Lesions were frequent; 34% seen with the naked eye, 49% seen with colposcopy and 52% seen with toluidine blue dye and subsequent colposcopy. The lesions lasted for several days; the median survival times for lacerations were 24, 40 and 80 h, respectively.
Conclusions

The legal implications of these findings are that genital lesions by themselves do not corroborate a rape complaint. Genital lesions may, however, corroborate specific details of a case and should be documented as carefully as any other lesion in rape complaints.

This was featured in an article here but seems to be a different study:

An in-depth study of 39 rape victims on one side and 110 nursing students on the other reveals that voluntary sex causes vaginal injuries just as frequently as in rapes.

“The findings are extremely interesting,” says Birgitte Schmidt Astrup, a doctor and a PhD student at the Institute of Forensic Medicine at the University of Southern Denmark.

“The nursing students experience just as frequent vaginal injuries as rape victims, and so these injuries cannot be used for much more than to establish that intercourse has taken place,” she says, adding that in cases where convictions have been based on such injuries, one can reasonably discuss whether there has been a miscarriage of justice. [...]

The results showed that vaginal injuries were found in 36 percent of rape victims at the Centre for Rape Victims at Odense University Hospital, and in 34 percent of the nursing students.

However:

The researcher is keen to point out that this is still only a hypothesis, which is also based on hints in other studies.

American researchers, for instance, have shown that there are four to five times as many injuries in white women as in women of other races, and this could be due to a difference in the skin on the mucous membranes.

“It’s too early to come with a definitive answer because the research in this field has so far been very poor,” says Astrup.