Ah, Bella..
I wouldn't say the content with her seems rushed. Well, apart from that random first kiss - that kinda came out of nowhere.
At the end of episode one MC notices that even though she scolded him for his actions, she cared more about him than she let on.
Episode 5 kind of added to the image I had of Bella. I believe she had a wild side during her college time, partying and fooling around. I wouldn't say she went full hoe (I'm looking at you, Arieth!), but I definitely think she's had her fair share of fun. At least until she met her hubby/Jill's sister (I like that theory too) and things got a bit more serious. That's why she says "I know boys like you, you're only after one thing", she most likely had similar experience(s).
Later on, her husband/Jill's sister kinda contained her wilder side, or she learned to discipline herself for their sake, but she never really stopped being, well, her. Boy, just imagine all that crazy, wild, passionate sex
With that in mind, she was alone for the past 3 years, without having anyone to help her release all that wild/sexual energy, which is why she's so "weak" to MC's advances. And MC apparently picked up on that which is why he's more bold with her than with other main LIs. I got that feeling even before I started thinking about it.
Yes, she's married, and she showed that cheating on her husband does pose a big deal for her, but the craving is too strong to fight. Plus, we don't know what the hell happened, and why James left/on which terms, so maybe, deep down, she feels like he ain't coming back and that's why she is more open towards MC.
There's a lot more I have on this, but I don't want to bore y'all to death