Your parents have some beef they need to settle between themselves - not with you. Any parent requesting their kids to retaliate against the other parent is a red flag. It's manipulation. Beware of her !

Now, if your dad had done something to you and you felt like cutting ties, or perhaps your dad represents a serious threat to your mother and cutting ties is necessary for her safety, well that's a different case scenario. But that's not what you are telling us.

TLDR: you are not a bad person.

Okay so... your dad is unequivocally a piece of shit. You said he's like your best friend, but are you okay with your best friend being a piece of shit? There need to be social consequences for being an unapologetic piece of shit (which one would need to be to have an affair with their personal assistant and then move in with her). Just business as usual isn't gonna cut it (think if instead of cheating he'd come out as a Nazi) and you would be condoning his actions if there aren't negative consequences of some form for this fiasco, though how much you escalate is up to you.

Edit: I have to say, the attitudes some of y'all have about parent-child relationships range from ungrateful to absolutely deplorable. Like, seriously if I heard "it's the parent's job to emotionally support their children, not the other way around" from someone in real life I wouldn't let that person within five miles within anyone I care about.

People sometimes cheat, not okay, but it happens. Most likely the marriage wasn't going so well, and he got some attention he wanted from someone else. Your mother is hurt and angry. But she does not have the right to deny your continued contact and bond with your dad.

There is no good reason mentioned in your post to end your relationship with your dad to end.

However, how the divorce is handled is more important in my opinion. If he in some major way makes sure she gets a bad unfair deal, then things are different. If he not only cheats, but also does not share in a fair manner, then he is really hurting your mother in a way that is harder to accept.

No. That doesn't make you a bad person. Frankly, I think it's awful that she's asking you to, but I imagine she's going through some things herself at the moment.

I think you have the right take here already- your relationship with your dad wasn't the one impacted.

When someone gives an ultimatum of "me or them" (a or b), the best choice is almost always never a. It is the least restrictive choice.

If there were an actual reason to not choose b, the best choice is likely neither.

I understand the argument that "she is struggling" but the moment they make their struggle your struggle they are choosing to spread the pain rather than deal with it. This is never acceptable in a parent child relationship, more so if the child is a minor.

I don't get how nothing changed for you? Maybe you're next if a hot new son comes along? Anyway, ofc she shouldn't demand that but it's totally understandable when you've been betrayed by your family. Really, go to her. She doesn't choose, but your dad is a major asshole that isn't honest with his closest people. It's not good for you to absorb that kind of life. It will punish you your entire life. Yeah. I'm giving you my opinion. You don't need to do what your mom says, but if you don't take care of her now she could be ruined forever. I don't know you, but I know that you deserve a better role model than someone that isn't man enough to admit he likes a younger girl. Also probably get you therapy. It is more important now than ever probably, and make sure your mom does too if she doesn't flip back. I'm not saying don't talk to dad anymore, but jesus christ he's got a new kid to be with, and you just got a job. I don't know your age but not stepping up now could be much more devastating than maturing too early. Just trust. People with no respect for themselves will never respect you and vice versa. You make tour own reality.

Why is your mom the one that moved out? Also, the affaire partner moved in?

He's your best friend, so you three like hang out and watch movies together as a new family while your mom sulks in an appartment alone?

Not only does your dad suck but so do you. Maybe one day, you will get cheated on and understand the betrayal it feels like. Hopefully, you wont have children to drive the knife in even deeper. Relationships are suppose to change with a parent who has an affaire, it's abnormal not to be angry at what he did to your mother.

When you choose to forgive someone you're not necessarily forgiving their mistake. It's not condoning, it's giving someone another chance to do better. Forgiving and protecting against risks isn't mutually exclusive, it just takes more effort to do both.