When the penis goes up into the vagina letting sperm find the eggs often causing pregnancy. Sex usually happens in the closet or anywhere when people from the opposite sex or same sex are "horny" see horny for detials.
Shit Eatin' Grin. A smile on the lips paired with an oblivious look in the eye. An SEG can be alcohol-induced, pure ignorance, or just not giving a crap what anyone else thinks.