Because of America's godawful sex education in our schools, many people think that you can just have sperm inside the vagina and can keep on doing it without getting pregnant.
the thing with birth control is that, even if you are using it, it's not a 100% chance you won't get pregnant, there is no birth control in this planet to confirms that it 100% prevents pregnancy tho yes, it can help with some preventions it's not 100% certain.
Also if sperm is inside the vagina for too long, it can cause a yeast infection.
PLEASE JUST USE A CONDOM INSTEAD!!!