Spoilers ahead for Star Wars: The Last Jedi. There is a disturbance in the Force. A ship that should not be sailing has left the docks on a galaxy far far away, and nothing will ever be the same. Star Wars: The Last Jedi officially made Reylo, the official Rey and Kylo Ren ship, a thing, and as much fun as ships are, there are plenty of reasons why Kylo and Rey getting together is a bad idea. For one thing, he's a literal murderer. Oh, and then there's also the small, but classic Star Wars obstacle: they might be related.
The Reylo appeal is not lost on anybody who saw The Last Jedi. Their Force connection, slow hand holding, and shirtless visits all add up to some pretty intense sexual tension. Then there's their last confrontation, where Kylo essentially murdered his master to save her and asked her to run away with him — swoon. Even the fact that they seem to end up on different sides of the Force, with Rey on the Light and Kylo on the Dark, only heightens the romantic stakes. However, putting their differing ideologies aside for a moment, there is one very key factor that makes their potential love disturbing, and that's incest.
The Star Wars fandom is nothing if not completely familiar with the possibility of incest in their romantic dreams. There were, in all likelihood, plenty of Luke and Leia shippers who were devastated to find out that the two were, in fact, not just related to Darth Vader, but brother and sister. One can only imagine how the fandom would react if they were to be put through another "surprise, the couple you rooted for and even watched kiss are actually related" twist. Which is why it is time to say, one and for all, that Kylo and Rey getting together is a horrible, no good, very bad idea. On the (very probable, but not definite) chance that Kylo was lying to Rey and Leia is actually Rey's mom, then it's probably best to keep this ship away from becoming actual cannon (i.e. be manifested in Episode IX). But the reasons of why Rey and Kylo shouldn't get together go beyond just the possibility of accidental incest.
Even though Kylo and Rey seemed to agree that her parents were just a couple of junk traders who sold her and abandoned her on Jakku, a lot of fans are still wondering if maybe Kylo was lying. Or, maybe the Force showed Kylo what he wanted to see, because he didn't want to know the truth: that Kylo and Rey are actually twins (or, at the very least siblings). Them being twins would really take the parallels between the original trilogy and the new trilogy to a new level, but it's also entirely possible that the two aren't twins at all, but just brother and sister.
Even if there wasn't a strong and vocal subset of fans who believe Rey and Kylo are related, their potential union would still be an awful idea. Kylo is a mass murderer. He's killed countless of innocent people since turning over to the Dark Side — one would be enough of a red flag as it is. Now, there are those that will say that Kylo can still be redeemed. And this may be true. Kylo still has an entire movie to decide to turn his back on the First Order and help Rey save the future of the galaxy, but does that really make it OK for her to simply forget all the pain and suffering he's caused and be with him? Redemption is nice, and it would certainly make him worthy of Rey's forgiveness, but it wouldn't make Kylo worthy of Rey's love.
Finally, there's also the small problem of Kylo's anger issues. His frequent violent outbursts are unpredictable and especially deadly due to his power with the Force. Any person in a relationship with Kylo could be at risk of becoming a victim of domestic violence. It might not be appropriate to say given the fantasy nature of Star Wars, but Kylo has abuser written all over him.
Rey closing the door on Kylo and their connection at the end of The Last Jedi was the right call, both for the fandom and for her. But, don't worry Reylo fans, you'll always have that hot shirtless scene and fan fiction.