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 jerking off, she's totally unfazed, because she has often walked
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.")
(page 6, panel 2)
Far from being embarrassed, Mom enters into the spirit of the moment by
graciously stripping & 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. ("Go on. Just pretend my hand belongs to her.")
(page 8, panel 3)
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.