Beat his ass

I would hate to have something like that come up. I tend to avoid confrontation as much as possible but telling him to stop and keeping it from my mom would feel like I was betraying her big time. Tbh I really don't know what I would do

Nothing, I'm not a moralistic little tattle tale. What she doesn't know can't hurt her.. there's nothing inherently wrong with 'cheating', the concept is just part of a retarded social paradigm. https://philosophy.inhahe.com/2018/06/21/cheating-fidelity-loyalty/

Blackmail time

Nothin, it's fair I guess, since they aren't married or in a relationship.

Find a suitable therapist for myself

Chop his balls off

I'd probably have a go at my dad first and tell him to get his shit together. And if he didn't, tell my mum.

At what age shall I assume this to happen?

It depends on how old I am. A child (was in this situation before)— I pray to God that they will soon figure it out on their own, but know deep down inside that even this won’t change anything. If I’m an adult— I tell her— even though deep down inside i know this won’t change anything.

I'd confront my dad about it.

Blackmail.