Nevri
I try really hard, but I'm not sure what you meant in that sentence ;/
If you mean the "It's far from impossible to find someone who won't be cheating on their spouse who doesn't want a serious relationship." part then it's just talking about open relationships and such. The part in parentheses is talking about how that sort of relationship may be more difficult to find in a place such as Japan which has different cultural values.
Imo Botan decision is totally understandable. She was forced to become a mom and working adult in order to support entire family all by herself, while still being pretty young. I don't remember all the details since I read it long ago, but father was gone since she was a child I think. She gave up going to university which she didn't want, but had to.
Her mother died when she was 15 and her dad left when she was 18 if I recall correctly. I just reread it earlier but I wasn't paying attention to the ages so I may be a bit off there.
I can see why not only she wouldn't want to date anyone seriously, but actually wanted to be spoiled. Sure, she could just date single guys if all she wanted was sex, but married guys are better for 2 reasons. 1. They are still less likely to get serious about her. After all not everyone immediately goes and break their marriage and some just like the trill of cheating (not that I approve of it, but that is the fact). 2. She actually wants a father figure, that will spoil her, as we can clearly see here (the first quote is pretty good too, hinting that you never know when things you take for the granted suddenly ends).
This sums Botan's stance pretty well
Don't get me wrong, I can understand why she would see it as the most appealing choice. I still completely disagree with it though. While an argument could be made that the person is going to cheat anyways even if it's not with her that doesn't really excuse that she's disregarding that person's family all while claiming to care for her own. And while I can understand why that might be kinda the last straw for her the way things turned out she was still rather out of line in leaving like that in my opinion, at least as permanently as it seems she did since a year later she hadn't even contacted them. She basically went and did exactly what their father did.
The thing is that while her siblings definitely did a piss poor job of explaining things it's pretty clear that they just didn't want her to harm other families but didn't want to make her out to be the bad guy either when she kinda was. Cheating can tear a family apart. And while arguments can be made that the person getting cheated on should have had at least some idea that something was wrong with the relationship (not always the case however because some people just cheat regardless of whether their relationship with their spouse is good or not) there is no argument to be made for the kid having to be pout through that. If it weren't for the fact the guy had a kid I wouldn't have had nearly so much of a problem with it honestly. I'd still think both Botan and the guy were pretty awful for knowingly doing it because the guy should just break up with his wife if it's that bad but I wouldn't consider Botan to be so much the bad guy. Regardless of her intent her fling with the guy drug an innocent kid into the mix.