I remember picking this book up at a library maybe 10 years ago - the story was about a boy and a girl who were (I believe) the last humans alive and were eventually expected to procreate to further the species. Luckily they liked eachother. It took place on a spaceship and I think there might have been some sort of robot guardian for them on the ship. I also remember it having a one-word title (not sure about that though). Anyone know what I'm talking about?