In pt II of g_bear's Wednesday-sex-thread-bonanza-based-on-articles-from-slate.com, I've just read that those women who have teh anal sex, are more likely to have an orgasm.
"Among women who had vaginal sex in their last encounter, the percentage who said they reached orgasm was 65. Among those who received oral sex, it was 81. But among those who had anal sex, it was 94."
http://www.slate.com/id/2270622/