A lot of women, when they get very excited when having sex, squeeze their inner vagina muscles which, in turn, bend the man's penis drastically like a boomerang. If he is not fully hard in the first place (possibly through nerves or excitement etc) then the chances are good that that squeezing motion will force him out of you. This is prevalent in the missionary position, so you might like to try other positions which allow his penis to curve upwards rather than downwards.