I remember reading about this somewhere. It wasn't more of a hypothesis than a theory, which needs to be backed up by some kind of scientific evidence. The psychologist who suggested this hypothesized that a woman's immune system views a fetus as being a threat. The idea is that this is because half of the fetus is made up of foreign genetic material. Therefore swallowing semen is making a woman's immune system used to the foreign material, kind of like getting a vaccine. There have been no tests to prove or disprove any of this. It all sounds rather far-fetched to me, but if someone has more information, please share it.