
It's not unnecessary at all, that's the point. She does love him, but she knows they need to make it clear that HE loves HER to get rid of all the rumors and any potential future issues that would be fueled by the rumors, like ladies thinking he doesn't love her and trying to become his mistress. If it's a well documented, established fact that the DUKE fell in love with and proposed to HER, that can be used to refute any scandal rumors in the future. The Saint's rumors were so devastating BECAUSE the Duke was known to not be attached to anyone and suddenly there were rumors of the saint coming out of this stoic guy's bedroom naked. If he'd been with Adele romantically at the time he could've just denied them and said he loves his wife and she could've supported him in denying he had an affair. That's the goal for the future. She's insisting on an extravagant proposal so that their words, their refutation of any future rumors, will have more stock. People will be quick to believe them and just dismiss rumors if it's apparent that the duke is head over heels for her.

Before I say anything, to avoid any besides-the-point "the age gap isn't even big!" replies, let me clarify that I have no issues with the age gap as they seem to be close in age anyways. My issue is how the manga relentlessly hits home that the MC is still very much a naive highschooler kid and I said this in my other comment below. But that's besides the point of this post, I want to address some comments/replies below:
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I see some replies/comments making the argument that he is mature because he takes care of his brother. Putting aside that child care doesn't teach you how to notice or handle someone trying to manipulate you emotionally or into sex, the manga makes it VERY, VERY clear that the MC is naive. He bases his views of romance on shoujo... he thinks the seme is like a shoujo prince and is genuinely shocked that he hadn't been kabedon'd when he tried to make the seme jealous... let's not ignore reality guys. It's also made abundantly clear that there is a huge experience gap here as the seme always calmly, collectedly flirts with the MC while smiling, while as the MC goes all doki-doki. It shows him getting mad when people imply he's a kid, he also hides that he reads shoujo from his brother to, and I quote, "maintain the cool big brother look". The story even makes his younger brother seem to be the more mature one when he tells his younger brother to beat up the other kid, like, twice... Okay, that's enough of that now, I'll go ahead and throw that argument in the trash for you guys.

They don't look that far apart in age, it's not the age gap that's an issue for me, it's all the emphasis the story has put on that the MC is a highschooler/still a kid. One of the seme's friends from when he went to high school said that to them highschoolers are so young. It's that he knows the MC is in high school and is actively pursuing him and flirting. Even asking him out and saying flirty things. I saw someone made a comparison to dating an older person in highschool and how highschoolers are attracted to older people, but to me it's weird that the older guy is the one initiating as there's a clear experience imbalance and just because a kid WANTS to have sex with/date you doesn't mean it's okay to. We as the adult need to understand that teenagers aren't fully mentally developed and will make more genital choices than they should. That's why waiting is a thing.

Yes EXACTLY. As a high schooler the age gap made me feel extremely uncomfortable even tho the couple was cute. Waiting until her graduate to start the story would've been just fine. Even waiting until he graduate from this point is kinda weird but 100% better than getting with a high school at your grown ass age.

What I found most awkward was when the seme's friend from highschool commented on how highschoolers are so youthful to them. Like, how do you not feel any guilt while someone you know that's around your age is saying that, the people you know are mentally at the point where highschoolers are just cute things they want to cheer on...

The ecchi was surprisingly not cringy? It's more of just a running gag. It was actually surprising how it was integrated into the story as just being a part of Rudy's personality. They did a good job of establishing it as part of his character and even gave it roots in Paul and the Greyrats' being horndogs. Can we talk about Paul for a second? Because what the fuck? His character is all over the place. At first he's a loving, doting dad with a happy family relationship, then suddenly with no warning he admits that the baby is probably his and he cheated, then he gets super dickish when he and Rudy reunite. I'm glad they worked things out, though the whole cheating thing was weird af and they all just sorta moved on. At this point idk if I should trust that he's been faithful or not since he's been separated from them for years. I'm so sad that those three died too, poor Eris. :^(

He's ACTUALLY a whore, and not in the "my body my rules" acceptable kinda way... he was in a relationship and just straight up cheated. Like, just break up if you're having doubts to that degree. Only at ch 8 so far, but he's so annoying so far, i hope his boyfriend leaves him and finds someone who can do <the bare minimum> by not slutting around on him.
http://www.mangago.zone/read-manga/the_justice_of_villainous_woman/uu/to_chapter-0/pg-1/
I laughed out loud multiple times. Especially when she wakes up looking derpy and startled in her new body! :,^D