I think this is turning into the eternal discussion of who's more or less to blame - the father/husband for betraying the family, or the the OW for going with a man who has to betray his family to love her.
Neither one comes out great. Both cause hurt in order to have their desire. Ultimately both are responsible. Both had the choice to say no, I'm not going there.
Beautifulmelody I totally hear you and thank you for speaking up with your irrespective as a child caught up in the effects of an affair. I don't think you're irrational - it makes sense that your father, whatever he does, is still (in a sometimes difficult and confusing way) your closest family. Of course you love him. Why should his sexual choice affect your love?
I use the word sexual here because that's what we are all skirting around. Beatifulmelody has said it. It's bad enough as a kid realising your parents have sex. But the horror of the thought of him having sex with another woman - it's actually traumatising. Especially if you're in your teens and kind of get what's going on.
The horror of that reality takes a long time to get over, for some children. (And wives but let's focus on the kids for now). And the sense of transgression/lawlessness - that a boundary that should never be crossed has just been demolished. We all know that if the OW in beautifulmelody's case had said no, then that boundary might not ever have been crossed. Her dad might still be with the family. Of course she detests the OW. It's completely normal and logical.
Kids would usually rather have their parents together with a bad sex life than apart having amazing sex with someone else. Of course.
I think the kids' feelings should be respected and they should be given time.