Science hates to be a buzzkill but often must - having sex with someone from the future might shorten your lifespan, thanks to antagonistic coevolution.
Writing in Evolution, researchers outline how they preserved previous generations of brine shrimp (Artemia franciscana) - A 20-year period, so over 160 generations - and then reanimated them to study coevolution in the species. It's possible because the shrimp come out of dormancy quite well.
They found that females who mated with a different generation had shorter lifespans - even on a short evolutionary timescale.
They had no problem mating, notes Jennifer Abbasi at Popular Science (who has fast become the most fun read over there) they just died younger. Antagonistic coevolution is how a species will adapt to each other to promote their own individual reproductive interests, sometimes in a sort of arms race ( see African bat bugs in Extreme Transsexuals In The Bug World) - that means if our guy showed up from 1811 with the evolutionary equivalent of a black powder musket, he might not die right away but it is likely to make him less likely to survive a war. And evolution is a war.
These were shrimp, of course, and only females at that, but antagonistic coevolution is likely occurring in humans of both sexes, even if we can't see it in a lab - there's a reason it takes a million sperm to fight its way through a vagina and ring that bell. It might only be more dangerous in future sex organs. So have sex with people from the future if you must, you have been warned, though I'd still avoid the hot girl vampires from outer space.
Mathilda May in Lifeforce, just one of the many, many, many examples of outer space creatures being hot girl vampires.
Citation: Nicolas O. Rode, Anne Charmantier, Thomas Lenormand, 'MALE-FEMALE COEVOLUTION IN THE WILD: EVIDENCE FROM A TIME SERIES IN ARTEMIA FRANCISCANA', Evolution, DOI: 10.1111/j.1558-5646.2011.01384.x
(1) Exception - Ben Franklin. That guy could do anything and, apparently, get anyone, so I would not be surprised if he would be the life of New York City even today.