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.
I grew up with The Legend of Zelda. "Well excuuuuuuse me, princess"