My boyfriend thinks having sex and having his semen in me one time you get pregnant over night i told him it doesn't work like that what should i do?

Pregnancy possible: Assuming that you and your boyfriend are both healthy and fertile, the chance of getting pregnant is about 20% with each intercourse. So the chance of not getting pregnant is 80%. So the odds are in your favor that you won't get pregnant but there is a chance. My recommendation is to gamble in Vegas and not in your bedroom. Birth control is free in many places. Good luck to you :)