I feel like I don't want to know my father anymore due to his infidelity. Do I just keep pretending?

I think you need to deal with this head on as its going to really start eating away at you, you need to see your Dad and get some answers off him, he does owe you an explanation about this affair. I think before anything else you need to see him, don't make any judgements, just listen to what he has to say, then say your piece. But you also need to speak to your Mum and tell her that you have to deal with this in your own way, and that this "pretence" isn't it!!! He has made a stupid mistake, and the fall out affects your whole family, but the big decision as to what happens next will rest with your Mum, as she will feel devasted, but she has the choice whether to forgive or not. I know that this will have ripped your family apart, your Mum will really need you now.