The first chapter of Larry Niven's Ringworld mentions this. At that point they have had transporter technology for a while, where you step on a pad and instantly you're somewhere else in the world. Opening up that level of ability to move around meant people mixed as well, and after a few generations everyone tended to be more similar than different. Probably not realistic, as there will still be culture and prejudices that influence pairing for a while, even if everywhere and everyone was just a step away.