you know, all that nasty stuff you wanna do to the opposite sex
your parents endulged in that w each others.
doggystyle, licking and sucking.
your mother probably swallowed, since thats a fairly common thing to do, between people in love
i bet she cried out "harder! harder!" and your dad, all sweatty, and invigorated by her request, went "yeah! take it deep!" thrusting her hard from behind, squeezing her boobs in his hands, maybe even reaching down over her belly, to fondle her while he thrust it deep
i love to give people that mental image