I mean even if he'll keep her as non-legal lover/mistress - that wont be good from political perspective because she'll be involved into court and noble machinations and schemes.
It will basically made her a target for various opportunists and schemers to use her against prince and royal family in various power struggles.
So his decision to keep her away kinda right.
Possibly, but I find it rather odd, particularly given the blatant polyamory in this culture. I'm going to say "Polyamory" because bigamy tends to be defined in rather sexist terms. In these stories, I'm an equal opportunity bigamist. ROFL. THE WIFU GETS TO HAVE MULTIPLE HUSBANDOS IF SHE WANTS! DEAL WITH IT, BOYS! ROFL.
But, anyhow, I get the logic behind saying that she'd be cast into a "complicated" scenario---I mean, she'd have
quite the shocking
wake-up discovering
her man is a noble, and not just
any noble, but
the seventh prince of her nation. ROFL. But, unless he just can't see her that way, isn't he disenfranchising her of that opportunity? Like, shouldn't she get to decide her own agency? Like, I get the standpoint that he really doesn't feel comfortable with ANY women his age, since they feel... Um...
underage. That makes sense. But just saying, "Nah. They can't cuz
commoner." feels HELLA classist. Like... Dude. I didn't take you for a snob. An outcast otaku NEET loner---definitely. But, a snob? That's unexpected. And contradictory to your character thus far. It makes me wonder what's his deal. And, yes, I do get the social pressures. But that's why I say, tell her the truth. I get he thinks that he'll have to keep everything a secret. But he's going to have to tell her one way or another. She'll eventually have to find out. And either way, she'll be at risk. Does he honestly believe that some nasty, nefarious person wouldn't willingly use her to blackmail him? Like, irrespective of them being in a relationship? SOMEBODY will eventually find out. And when that happens, what's he going to do? Leave her to his own devices? Anyone and everyone he becomes close to is at risk. And I KNOW this kid ain't frigging Batman. And even Batman ended up making matters a "family" affair. XD. There really isn't an alternative. He can "just be friends", but either way, she's going to be vulnerable. At least, if they are in some kind of formal relationship, even if she doesn't live in the palace most of the time [and works as a merchant; he's
ONLY the seventh prince after all; and this could be yet another excuse to not become king---we'll see what happens], as a spouse, she'd be more eligible for royal guards. And what merchant wouldn't benefit from that? :3
Plus, he's apt to get put into even worse situations if he doesn't think and plan ahead. Dude is asking for it at this rate. Who knows? He might end up in an even worse state like some other characters from other isekais in similar circumstances.