A staple of fantasy involving time travel is the hero (or heroine) zipping off into the future and then meeting a future lover with whom they go on to have wacky time-based adventures before presumably settling down to make some babies.
Eventually, though, this won't work. Even if one of the future people falls in love with our time traveler, they won't be genetically compatible because humans will have evolved to much in the interim. How far into the future would someone need to travel before running into a species barrier between themselves and the locals? I'm assuming that there haven't been any major evolutionary bottlenecks due to some sort of apocalypse, and that people haven't utilized technology to artificially speciate themselves or stabilize their genome and actively prevent it.