based on the concept that ninjas can kill you without you knowing they're there, ninja sex is the act of having sex wit a person without them even knowing your fly is undone! in this situation its generally useful to be very quick or poorly endowed. both of which are traits known to be commonplace amongst ninjas oddly enough.
i was out the other night and stayed over in this girls place.decided i was too tired for a proper ride so i decided on a bit of ninja sex we were making out a bit and i got to taking out my dick, replaced my finger with it, blew my load tuk it out and got the finger bak in without her knowing. she asked the next morning why we didn't have sex. i got mine! ninjad!