Digging out an old journal and an older idea. He's a time traveler from the distant, decadent future, traveling through time for fun. Along the way, she--a native to the time period--catches his eye and he decides that, really, some sex would be great.
I'm thinking that he's probably got some technology to impress/woo/arouse her with. Open to the time period she's from, but the 1950s would be a lot of fun.
Don't necessarily have to use Matt Smith. Comments screened.