what's your opinions on this

had sex with my boyfriend and we used withdrawal (don't judge or tell me we should use condoms, we don't and haven't got pregnant in the two years we've been using the method). after a few we had sex again. he didn't cum the second time. 
I've heard that if the guy doesn't pee after sex and then has sex again then there could be leftover sperm in there and they could get pushed out and into the vagina. 
what are your opinions on this? does anyone think they ever got pregnant this way?