So that's why her girlfriend allows to get fucked by his uncle balls deep? Lmao, I wanna remind that the whole family hated his uncle or something while that and in the instant that this said uncle gets his eyes on the girlfriend of the single one family member that liked him, INSTANTLY gets his problems on getting his dick up resolved
Like fuck, imagine not being in the mood for one night and your girlfriend immediately jumps on the closest dick near her, Satoshi definitely had some his issues but his girl is a slut bro
I think that Kendo getting his virility back at the mere sight of Mary was more supposed to be for comedic relief, cause that was kinda hilarious. Also, liked? Satoshi takes every opportunity to put down Kendo, whether it's his weight, his age, his carefree attitude, or his playboy tendencies. Maybe it was different when he was a kid, but from what I've seen, grown-up Satoshi gets easily irritated with Kendo.
Regarding Mary, she isn't the kind of person who will jump on some random dick if Satoshi isn't feeling it for one night, at least initially. Prior to the resort, they only had sex once, and they've been dating for about a year. Mary didn't get to the point where she's willing fucking Kendo because of one thing. It was little things that began piling up. Satoshi not wanting to spend time with his girlfriend if it involves something SHE's interested in while he's not. Satoshi not taking good enough care of his body (Dude stayed up all night looking at OPTIONAL classes when the fucker knew he needed to be up early to go on a massage retreat with his girlfriend). Satoshi not willing to accept criticism properly until it's far too late and the damage is already done. Until finally, Satoshi explodes on his girlfriend when she wants to express her love through sex.
I'm not saying Mary is free of blame, but Mary didn't start becoming a 'slut' until the HJ scene with Kendo (Brain rewiring to seek greater sources of pleasure and all that). If Satoshi stepped up to the plate like he should have before the massage retreat, I think Mary would be more than willing to cut ties with Kendo, especially if Satoshi proves to her that he's willing and capable of being a better boyfriend. She did in the first Satoshi Ending, provided you make Satoshi apologize to her, showing that the root of the problem lies with him.
*exhale* Sorry, I'm a bit long winded

And I've only recently played past Mary's first few sex scenes with Kendo, so I have no idea where the story may go from here. Just voicing my thoughts as I'm going through for the first time