There are apparently no factors other than sex to explain it:
http://www.cnn.com/2014/03/13/health...html?hpt=hp_t2
Key quote:
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The couple said they routinely had unprotected sexual contact and shared sex toys between them. At times, the contact was "rough to the point of inducing bleeding in either woman," according to the CDC. The women said some of the unprotected sexual contact occurred during menstruation.
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Not even lesbians are safe.