TBH It's probably because this is really a Netori. This has no loss or feeling of someone being cheated on. It could basically be viewed as a vanilla hentai. That's probably why you like it
Nothing is sweeter than a woman (especially a MILF) who's totally cool with
male masturbation. When Mom (we never learn her name) catches Keisuke
with his long cock in his hand, she's totally unfazed, probably because she's
used to walking in on her son during his own "boy time"---and not always by
accident, I suspect. ("Tatsuya yells at me quite a bit about that.")
Far from being embarrassed, Mom enters into the spirit of the moment by
politely stripping and inviting Keisuke to keep fapping while she watches.
When he hesitates, she lends a helping hand---totally unfazed, once again,
by the fact that he's fapping to a loli video. She even involves herself in his
fantasy. ("Just pretend my hand belongs to her.") Whatever turns Keisuke
on is okay with Mom. Who wouldn't marry a good sport like her?
Dude, you don't just offer to put your buddy's mother up for the weekend and try and retract it when she does arrive! Especially when she's hot like her. Also, I'm not usually a fan of cheating releases, but she did mention her husband cheated on her before too...And hey, she actually got a divorce with the guy and married our protagonist instead! So I'm perfectly happy with how this release ended, truly.
And so, he went from being his friend's...er, friend, to new stepdad. And it seems that he has been converted from a lolicon to a MILF lover, as signified by him handing his...new stepson his prized loli DVD. And hey, it's good to see his stepson being so accepting of his new friend turned stepfather. I think they'll have a really interesting family life.