If you are talking about when you are not pregnant, the cervix is closed but not sealed. It isn't gaping open, but sperm are tiny and can get in. When you are ovulating, the os (the opening of the cervix) does gape open a little, and the more friendly eggwhite cervical mucus comes out. So things are generally more welcoming for a lot of reasons, when you ovulate. But sperm can get in even when you are not.
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