What are the odds that a girl would get pregnant if he ejaculated then stuck his penis in her vagina?

The odds? About 1 to 5% chance. Depends if the girl is in her fertile period. The short end of the story is that some people can do it 2000 times and never fall pregnant. And some people can do it just once, and they conceive. You really cannot tell. But you are risking a lot if you continue to do it!