Always enjoy reading and learning from your comments..It depends. First, your cervix becomes tighter and looser depending on your time of month. Second, it depends on angles and just how he is aimed. Obviously, if he goes in at a sharp angle and doesn't get close to your cervix, he can't get anything through it. Third, there is relative size. The mechanism is that a large flare will seal inside the vagina and the ejaculate won't have any other place to go. Actresses have insisted on using condoms claiming that multiple sessions leave them bloated.