"Hello, Anachronisms R Us... what can we do for you yesterday?"
Time travel historical romances, science fiction romances featuring time travel... time travel in general is pretty popular in the erotic romance genre. It allows the main characters from the modern day world to find themselves swept off to medieval Scotland, ancient Egypt, a future Earth--and many other romantic destinations. It allows readers to escape and wonder what living in that period was like. Does it have romantic or action elements you wouldn’t have otherwise, such as men fighting with swords or dueling with pistols at high noon? Or does it sweep you away to a far-off world in the future where the imagination can run wild as to what human beings have done and can accomplish? After all, where else can you read about having sex in zero gravity?
One of the things I like best about time travel romances is that it can span genres. Books involving time travel can be fantasy, science fiction, and/or paranormal works. They can even be more mainstream in genre and have time travel as the sole paranormal element.
In terms of the speculative fiction genres, I also greatly enjoy alternate history as a basis for developing characters and setting. This is part of why I think urban fantasy does so well; it’s a “what if” scenario where every day elements are mixed in with the fantastical, allowing the development of a whole new world that looks just like our own.
My favorite time travel series is sadly not a romance, but has held romantic plots as of late: Doctor Who and Torchwood. Fans of science fiction programming should check them out--especially if you like m/m and f/f as Torchwood contains elements of both.