It really is! I've talked about it at length with them because I feel the potential to get messy is huge. But they've got a system with vetting potential partners and are completely honest with anyone who wants to get involved. Like, one of the rules is that the partner can't be in a relationship (doesn't matter if poly/open) as that means unknown people who can complicate things. It also wasn't easy at first because they didn't know she was asexual to begin with, so there was so much insecurity with him thinking she didn't love him, and her questioning what was wrong with her.
Ohhh that's really nice because I'm also asexual (and aromantic so idk if it applies since I don't really want a SO) but that kinda dynamic is exactly what I have in mind if I hypothetically ever get into a relationship. I don't want my partner to be deprived of their needs just because of me. Of course it'd be best if I also had an aroace partner but if not, that dynamic is what I'd want. As long as we're both honest about our relations and the other person is someone we both can trust it's ok.

I know a couple who have the same exact dynamic as Miho and Nao. She's asexual, he's bi with a really strong sexual drive. They have rules in place of course: she knows all his partners, when they're going to do it, and only with males (apparently just to avoid any unwanted pregnancies/complications). As far as I know, they've had no problems with feelings getting involved from either parties. They've been together for 8 years, and they're actually getting married this year!