Strong swimmers...

So, the males in my boyfriends family have very strong swimmers. Him and his sisters were born even though their mom was on birth control. His dad and his siblings were born even on birth control. This goes back as long as they can remember. For 9 months, we've had successful sex with condoms. I want to be on bc so We don't have to use condoms anymore, but we're scared to. Today we bought the Today Sponge and are wondering the effectiveness of it, especially if we pull out.