You're doing fine Miller, but you don't have to devil's advocate any more than you want to.
Science fiction does, actually, have to have a science explanation for its story. Science fiction are stories of imagined realities for which there is a scientific basis. (Hence, the science in the title.) Of course, quite often that basis is pretty shaky and speculative, but the important thing is that it is there. There are a number of areas where people seem to get confused over sf versus fantasy, such as:
1) Psychics -- we don't know if psychic abilities such as telepathy are possible. Currently, they are not and there is no scientific proof of their existence, though many people believe they exist. It is conceivable, however, that alien life forms might have psychic abilities or that it might one day be possible to genetically, chemically or otherwise scientifically develop such abilities in humans or other creatures. Therefore, you can have a science fiction story about psychics, if the psychic abilities are due to scientific speculation. You can also, though, have a fantasy story about psychics who have their abilities from magic or the paranormal.
2) Vampires, werewolves and the like -- vampires are a fantasy legend and usually have their abilities and behavior controlled by magical or supernatural forces, such as an aversion to Christian crosses and such. But it is also possible to turn vampirism scientific, by casting it as a disease, an alien life form, or a genetic mutation. If the vampires are given a scientific explanation for existence, then you have a sf story. If not, it remains a fantasy story. Same for werefolk and all the rest.
3) Time travel/alternate dimensions -- there is a whole sub-genre of sf called alternate history which postulates alternate timelines and sometimes travel between timelines. There are also stories about time travel through the use of machines or drugs. These stories are based on speculation from quantum physics, string and chaos theories and such. Though we don't know that any of it is possible, and indeed, it may be impossible, it is scientific speculation, and as such, sf.
There are also, though, fantasy stories in which a person moves through time or alternate dimensions by means of some form of magic, a system that is magical or mystical, like a handy dandy crystal. Fantasy stories in which a person from our current world is transported to a fantasy realm where magic works, for instance, were extremely popular in the 1980's and still come up today. Merlin, the most famous wizard, lived life backwards. King's Dark Tower series is a fantasy story in which the characters move through magic from one realm to the next, including regular modern realms and, I believe, futuristic ones. In the fantasy stories, though, there isn't a scientific basis present for being able to move through time, space or dimension. King meant the stories to be considered fantasy, but did enjoy throwing in sf-like elements. There are also of course many fantasy stories that take place in imaginary worlds, especially many of the epic fantasies. These can be seen as the equivalent of alternate dimensions, but since no scientific explanation is given for their existence, and most of them contain magical or paranormal phenomena, they cannot be sf.
4) Traditional elements -- If a fantasy story has machines in it or is set in a futuristic setting, people tend to confuse it with sf. But really, there isn't a sacred law (written in diamond on Mt. Creativity,) that says a fantasy story can't have trains, spaceships, science issues, robotic polar bears or anything else like that. Likewise, a sf story can be set in a historical period, involve swords, talking animals, and the like, as long as there is a science-based explanation for the elements of the story. There is a sub-genre of sf that is called, badly, science fantasy, meaning sf stories that have the look and feel of fantasy, but give it a science basis, such as Anne McCaffrey's genetically engineered dragons on a colony planet (with psychic powers no less.)
As long as the story has magic, the supernatural or the unexplained fantastic, it's fantasy. If all phenomena is science-based, however theoretical, it's science fiction. SF has a number of sub-genres in the market. Hard sf features stories involving hard sciences such as chemistry, biology and physics, whereas soft/sociological sf features social sciences such as anthropology and psychology. Space opera are adventures set in a sf imagined future.
There is apparently a movement called mundane sf which feels that the only really worthy sf stories are those that do not involve space travel or alien life forms but concentrate only on a future Earth, only on very non-speculative, hard science elements. And so that may be what was meant by hard sf versus fantasy sf. Or it may be that they don't like the science fantasy sub-genre, though it seems a rather big brush to tar everybody with. But it's a separate issue from the fantasy genre which has its own parameters and its own sub-genres.
Essentially, fantasy can have science in it, (and westerns, mystery, romance,) but must have fantasy in it. SF has no fantasy in it, only science. Stories that are historical but have no fantasy or scientifically imagined realities such as alien life forms are historical.