In Ethics class (I live in Spain), about WHEN I WAS 17 or something, the teacher told us that anal sex wasn´t completely safe since sometimes, it caused a wound that perforated the flesh between the anus and the vagina, thus the semen went into it.
I have a strange love/hate relationship with the memory of that teacher. She was nastier than a 4channer, without the cheap resort of insulting. And fuck, when I watched today "Teaching Mrs. Tingle", I was instantly reminded of her.
In a way, she was awesome. In another, she was a total bitch.