This comes from the popular word couch potato, basically describing someone who is lazy, sitting on the couch and watching TV (yes, without bothering to entertain the TV set once he's done)
Mary: God, that must be like slavery! Glad you left him. Date an athlete this time
Rob: "No, Last I heard they were at home having sex, but that was 2 days ago"
Laura: "What a bunch of sexpotatoes!"
person number two: really? how was it?
person number one: awful. she was such a sex potato.
Dude: Bad! Man she was hot, but when we got down to it she was a total sex potato! She just laid there!