A sperm sandwich is the act of a man ejaculating between a woman's tits and then eating it. (Spread is optional).
Man 1: Duuude I totally Sperm Sandwiched your sister last night.
Man 2: I wouldnt mind spreading some butter between her slices.
Man 1: Dude this is your sister you sick bastard
Man 2: Yeh but she's well slack