Cheating father with the maid

No. You would be using his 'mistake' as a weapon against him to get your own way.  If you really want to get married eventually you will.  A parent is a human being subject to human error.  Because you caught him 'cheating' is no reason to forget the fact that he is still your parent and only wants what he considers to be the best for you.  Neither does he owe you an explaination for his indiscresion.  That is between him and his wife.  You could tell him how you feel about the situation and ask him to help you with those feelings so that it would be easier for you to accept it for what it is - a mistake. Never hold it against him or try to use it as a weapon.  He is your DAD!

I agree talk to him and see where he's coming from.  And if you are over 18 there is no reason your parents have a say in who you marry.

There might be a good reason why they're against the that you need to resolve.

Ron

Citrines answer is very good.

Your father is human.  Humans are NOT perfect, not even your father or your mother.  Don't try to hold them up to be or test them by a standard of PERFECTION.

You do not know all the facts and circumstances about their marriage and all of what was going on with your father ... and you never will.

You should talk to him and let him know it is very disturbing to you and then let it go after that.   He is probably very regretful and very embarrassed.  Don't let this mistake ruin things for you and your father.  

Rob