Maybe it is not that serious? But it feels so important
And it seems like he is taking his autonomy away by saying he can not even pleasure himself ┑(￣Д ￣)┍
Your observations are quite valid and I understand your concern, especially regarding bodily autonomy. However, these are established marital customs of the North, not rules Ian personally invented. Northern society has its own expectations surrounding marriage, fidelity, and intimacy, and Ain/Ian genuinely observes these customs himself while also teaching Michele about them. For instance, one of the major concerns underlying some of their marital conventions is the birth of illegitimate children, who are regarded as “sins” that shouldn't exist.
Unfortunately, in the North, that's an absolute NO-NO. But you'll see their intimate dynamic change considerably as their relationship heals and develops. Their intimacy becomes increasingly driven by genuine love, desire, and passion rather than simply being governed by marital customs and obligations. Their wedding night was a whole steamy arc (▰˘◡˘▰)
I think it's important in the way that it reflects how Ain is thinking. Like he is clearly so caught up in trying to fit in with northeners that he's very particular about upholding their roles and customs. Which btw, might not even be as strict as Ain implied. I don't see why Michelle should believe in this strict anti-masturbation tradition when Ain lied so easily about his identity. To me it seems more likely that it's a mix of Ain, as I said, being obsessed when it comes to his reputation as a northern lord + something like jealousy (as in he doesn't like the idea that Michelle might be getting off to someone else), or further seeking control over him.
“You must do this you must do that” dude even in my manhwa they find a way to torture me with roles shit. Just do it because you love each other not because you have to. It makes the humans feel like sex machines or something instead of a partner （/TДT)/