It seems to me that such an assertion might rest on at least three assumptions:
1 that the sex is penetrative (cultural);
2 that the sex is heterosexual (cultural);
3 that women's bodies - as depicted by male gynaecological text books are 'weak' and 'at risk' (cultural).
Sorry to sound so serious and poststructuralist. Sex, if nothing else, should be fun.
But doesn't it seem strange that the same bodies that are capable of bearing children (with all the stress, effort, pain and punishment that it involves) are also deemed to be 'weak', 'open to contamination', 'delicate', etc?