Rumours aside, as a woman I can see why she would be having doubts about marrying him, on a superficial level (seeing as that's how these people operate). Jamie is a walking ick; the hairline, the dad bod, Ross Gellar glow-in-the-dark teeth, and OTT ADHD levels of energy, which must be exhausting and exasperating to live with and wake up to for the rest of your life. Probably harsh-sounding but I don't get a nice vibe from Jamie at all after how rude he was to my friend during her work event. I think a lot of his "cool, relatable guy" act is the public persona he loves to push as his career hinges on it.
Again, even before all these rumours started, it was obvious from their wedding photos how massively icked out she is by him. C'mon, it doesn't take a behavioural psychologist to work this out! Just look at her body language in the Spain wedding pics. He's trying to hold her close but she's pulling away and distancing her body from him, clearly signalling that she's not sexually attracted to the bloke. The London pics are just as disingenuous but in a different way. She's more focused on (bizarrely) posing for the camera while he's just a mere side piece. Look how miserable she is in the pic of her in the car with her dad. If that doesn't scream "regret", I don't know what does.