Options 3 and 4. I would urge you not to do that, Hagrid. Imagine how you would feel if your SO cheated on you, too - not just in general. Be honest if the relationship isn't as good as you wanted it to.
(I do believe that victims of abuse have a good reason, though.)
FTR, I am not considering cheating. I am interested in gauging whether this is a common thing and deciding if my dad is a complete SOB or just a bit of one.
Oh, I misread the OP - sorry. But I don't think we determine if something is right or wrong based on how often it occurs. I also don't think there's much you can do about your father's cheating, unfortunately. That has to be worked out between him and your mom. I really hope things turn out best for you and your family, though.
I'm also going to call BS on Brown Line's claim unless he has something to back it up...