Before Kara and Alex ended up fighting Nazi's on both Earth-1 and Earth-X in the "Crisis on Earth-X" crossover, Mon-El returned to National City. But it wasn't the reunion fans of the "Karamel" pairing had hoped for. Mon-El revealed that he had not only been gone for seven years, but was married now too. While many fans can't believe that Mon-El would ever marry anyone but Kara, there's a fan theory that thinks that marriage might just be a sham.
In the Supergirl episode "Wake Up", Kara Danvers/Supergirl (Melissa Benoist) discovers that Mon-El (Chris Wood) is alive and well, finding him in the mysterious space ship in the National City waterfront. But Mon-El is not quite himself, and as the episode closes out we see why: one of the other people in the ship's life support pods turns out to be a woman named Imra Ardeen (Amy Jackson) and Mon-El reveals that she is his wife. Naturally, Kara is shocked and heartbroken, but a fan theory that has shown up on Reddit and other places thinks that things might not be quite what they appear, claiming that Imra may be manipulating Mon-El's mind to believe that he is married to her.
The theory has some surprisingly convincing evidence for why this marriage could all be fiction. First, Amy Jackson herself has made some comments on social media that indicate there is a lot more to the Mon-El/Imra story than it looked like at the end of "Wake Up." Specifically, Jackson replied to various fans upset about the marriage reveal with comments telling them things get better. She specifically told one upset fan that there was more to the story. The responses were made to fans of "Karamel" and not just generally, leading some to believe that the marriage storyline will correct itself soon.
How it would correct itself is something that fans had further speculation on. In comics, Imra Ardeen is Saturn Girl, the leader of the Legion of Super-Heroes. She also has immense psychic and telepathic powers and in comics she has used them on her own team before. Following the Blight invasion, Saturn Girl and several other Legionnaires were lost in a galaxy not their own. In an attempt to keep the team calm and focused, Saturn Girl created a telepathic illusion of Ultra Boy's wife, Apparition, making it appear that she was there with the team. When Saturn Girl was put under for medical attention, the team realized that she had been manipulating them.
This history of manipulation -- specifically that Saturn Girl manipulated Ultra Boy to believe his wife was there when it was really Saturn Girl -- might be how Mon-El is "married" on Supergirl. While the theory didn't explain what Imra's motive would be, if she has the same powers as she does in comics, she would certainly be able to trick Mon-El into a relationship by using said powers. The theory points out that in comics, Saturn Girl is a huge fan of Superman, so if they transfer that idolization to Supergirl on the show, Imra may have seen marrying Mon-El as the closest thing she could get to knowing her hero. The theory also says that Mon-El still wearing Kara's necklace is proof or at least foreshadowing that something isn't right with the marriage to Imra, possibly something in his subconscious that won't let him take it off.
Whether or not Mon-El's marriage is the real deal and what, if anything Imra is up to, fans will just have to keep tuning in to Supergirl to find out.
Supergirl airs Mondays at 8/7c on The CW.