you're forgetting when Matsu is chloroformed n kidnapped by Tomoe's mom, n if he knows what's good for him, he agrees to marry Tomoe (for his family's safety, IIRC). then there's also Fuyu's mom, who runs the company that funds the school (?); Matsu has to prove to her that he's good enough for Fuyu, otherwise same thing (remember the 2 girls watching the house)
then there's Tanami, the school's principle (n Matsu's boss), n Crazy Nurse...i don't think either of them would take too kindly to having Matsu stop giving them the 'D'; so unless he wants to lose his job (n worse), pretty sure he needs to keep them happy. not to mention Satomi, 1 of his students, is Tanami's daughter; n with her dad (Tanami's husband) being the head of the school board, if he starts rebuffing her flirting n advances, she could drag his ball sack over broken glass.
and don't foget the Oppai Club...unless you played the MC as a sissified cuckold (which there's no way he should be, given his predator tendencies n MMA background), he has a binding, contractual business relationship w/Itsumi...he's the main attraction for the club's female patrons.
oh yeah...i almost forgot those 2 pain-in-the-ass students; the 1 who keeps taking photos of Matsu having fun, n blackmailing him, n that scooby-doo bitch Alice, who keeps coming way too close to catching Matsu w/his hand in the cookie jar...she's gonna catch him sooner or later, n then he'll have to "persuade" her to not get him in trouble also.
so yeah, i'd say some of the cheating is "forced", although i don't think Matsu's conscience is bothering him about any of it.
"...if there was enough sex at home"; SERIOUSLY??!! he's fucking Mao, Saki, n has done mutual oral w/Himiko (dunno why BC dragged that one out so Matsu still hasn't fucked her yet), n now he has his wife's sister (Kotori) living w/him that he'll obviously start fucking in the near future...IF this ever gets going again. not to mention there was at least once (twice ?) when Matsu checked on Yuki before going to bed, n starts "thinking" about things; so who knows there.
so yeah, i'd say there's plenty of content to satisfy your 2 points of interest/contention as to whether or not you've "wasted" your time playing this. n don't get what you mean by "Don't suppose there is even a point to playing the game if MC doesn't cheat outside the family"...he's fucking 2 daughters, about to fuck a 3rd, gonna fuck his wife's sister if he can ever come up with enough panini sandwiches, n might even move in on his 4th daughter; PLUS the waifu (who's WAY too nice IMO). i'm afraid she will suffer permanent psychosis once she finds out Matsu is stepping out on her, although her seemingly total self-worth n dependency on him might prevent that from happening...who knows; BECAUSE THIS IS NEVER GONNA GET FINISHED!!!