I think this is like that whole myth about the hole in the sheet. It's not really reality but people still keep to be passing it on.

When I took kallah classes before I married, I was told that when having intercourse with your spouse, it's a spiritual connection between you and your spouse and God. That's why you can only be thinking about your spouse and not some "image" you may have seen earlier (for example, if you saw a picture of a naked woman right before having sex. According to the rabbi's wife, it's a sin if a child is conceived during that sexual episode if both partners weren't thinking about each other).