Yeah, but they are supposed to be "in love", so we can give that a pass.
You can, but I'd call it "debatable", at best. And even if you do give it a pass, he's still casually fucking pretty much every woman with a pulse without telling her; by definition, he's cheating, if you make those choices. If you don't, it's irrelevant, but if you don't make choices that result in a MMF route, or the "pet" stuff, then they won't happen either, and are equally irrelevant.
So yes, he's not half decent, but at least he's not supposed to be a degenerate.
Again, he's fucking his daughter, after coercing her into a sexual relationship; he's already a degenerate.
And to answer your question, I wouldn't "let" my daughter fuck anyone, because it's not my decision to make. Are there some choices she could make that would enrage me, or disappoint me, or sicken me? Absolutely, but again, you don't "let" other people make their own decisions; you can only express your preference, and tell them how certain choices might make you feel, and then deal with those decisions and the consequences thereof.
Is this the game we were sold at the start? Fuck no.
I'm pretty sure I remember the Bestiality tag on Patreon like . . . I want to say three years ago? Was it as long as that? Back when the game first started, anyway; it's been there from the start, though I will acknowledge it wasn't brought up in game, and I kind of hoped he'd forgotten about it. So yes, it's absolutely the game people were sold at the start; they might not have read the small print, but it was there.
It's also worth noting that, to the best of my knowledge, the "pet" route hasn't actually been confirmed to include D, though it's certainly implied.
And as for the Sharing stuff, that's been there just as long; there are literal "Sharing Points" in the game, and the walkthrough has blatantly mentioned the possibility of MMF routes before.
Does people have a right to complain about it? Of course.
People have a right to complain about absolutely anything, and I'm not saying otherwise, but some complaints are definitely a bit sillier than others; "I don't like this thing being in the game, even though we will absolutely be able to choose not to experence it, and even though it was included in the description when the game began" is a less than stellar position for a complaint, in my opinion; your mileage may vary, of course. And if some people deem a complaint to be silly, they're absolutely allowed to say so, too.