I can't vouch for their quality, because I have not read these, but the following seem to meet your criteria and are currently offered as time-limited promotional freebies in the B&N store, courtesy of erotic romance publisher Samhain.
Hara's Legacy, by Bianca d'Arc. A multi-brother ménage relationship fights off an alien invasion. First in a series.
The Sword Lord, by Robert Leader. People from a doomed planet escape to colonize ours, landing in ancient India where apparently they end up being gods. This one has a warning for rape. Again, first in a series.
You may also want to have a look at romantic erotica publisher (their offerings tend to be slightly more explicit than Samhain's, though Samhain has much funnier content warnings) Ellora's Cave's e-bookstore, which offers their "Naughty Nooners" short stories in the
Free Reads section.
These are of varying quality, but they do have a number of science fiction and traditional fantasy (wizards, fairies, etc.) stories among the lot, and often the writer will also have a couple of books in the same setting, and it's a good way to try before you buy.
Finally, Circlet Press specializes in doing themed anthologies of erotica with fantastical or science-fictional twists (a little on the steampunk side, but still a fair number of space-based/future-tech/alien contact offerings). They put out a bunch of free stories as a Christmas promo last year, and the ones I read were generally of pretty good quality, as erotica goes.
Try a few of their
Free Reads and their "
microfiction" stories; see if there's an author who does stuff you like and find out which anthologies include their stories.
Probably the cheapest way to buy them is via
Fictionwise, during discount coupon sales (there was a 50% off coupon posted recently in the Deals forum; they seem to do this every so often, and have a big Xmas sale too)
Hope this helps, and welcome to MobileRead!
ETA: Well, now I'm up to 666, posting number of the Beast! Seems oddly appropriate that it would be this one.