Every update Monica's character gets worse and worse, and yeah it certainly does imply she's been with other men while they've been married, cheating or otherwise.
I would say she's been lying from the start but...and no offence to the dev here but I get the feeling he has been "winging it" with some of these personalities, and so characters don't stay consistent. Could also be he didn't want people to think she was the main LI so he's full on trashed the character. (A better option would have been to not make the early stuff so Monica focused and give an option to kick her to the curb.)
I agree...and the lies don't make her more appealing. And cheating and lieing are two traits that a love interest displays are not appealing to me. Regardless if it's happeing to some other dude...you learn she still wants to be with the husband. Also to me Monica's relationship has evolved into a sharing/swinging path...and it's favoring her more...which is not cool. And the way it's presented, I feel she fed the player bits of misinformation to manipulate him...another huge turn off (but MC accepts it...because he's thinking with the wrong head).
And one minute she tells MC she loves and want to be with him then tells him but she is only interested sexual intimacy, and has no desire to leave her husband because she loves him more than MC"...(and at that point we still didn't know about the hubby's condition. So we are left with a WTF feeling, "what, I'm not number one?") This ticks the dislike for Monica box even more for me.
And including...Monica asked MC on their third meeting I believe, she wants MC to find another female for them to have sex with. Again it seemed like MC is only being used as a human sex toy...or mabe baby maker as well for her and her husband... Which the husband mentioned during MC's second in person meeting with him, that he also provides her oral and probe sex (finders and toys). And most likely is doing much more of it NOW, especially now he works from home and knows what is happening to his wife when she is alone with the MC (and maybe doing it more to stay relivent, so he doesn't furthur lose her to MC?).
So for me at the relationship with Monica choice...I bit the bullet and went with the diabolical route. I'll Netori the shit out of the husband, and take his Monica away from him...

But I noticed during the in person converstaion, it's obvious the husband isn't a cuck. He is forced to accept what ever his wife wants, in order to keep her happy (he doesn't like it...but he has no choice or she'll sneak off and cheat regardless...like she has already been doing. And the husband also doesn't want to push her more into MC's arms...due to the friction his disaproval could cause...so fuck away, just don't take her away from me).
But right now it looks like this story is focusing on keeping a lot of key information hidden, or draging it slowly out with regards to Monica to create false preceptions. But the setup for both girls (Monica and Azumi) "I love him BS, and I wont leave him"...is odd for a Netori or harem path.
And then MC has the job to work on convincing (manipulating) both girls to stop being in love with their partners is...

. And yet, both men will be still sleeping right next to their partners (and I find it hard to believe nothing will be happening at all. Considering most know in real life...what happens when a man rub's his woman's back and ass while she is sleeping / just waking or going to bed...you get laid.)
Regards