It's not necessarily about doing something "wrong". In my fraternity the vast majority of guys we cut is because they don't fit in with the fraternity, not because they do something wrong. Rush isn't some checklist of mishaps you have to avoid (hitting on girlfriends, getting too drunk, etc) and then you get a bid. You also have to fit in with the house personality.
I think the fact that your father actively donates furthers this argument. I know in my house we give a lot of extra consideration to legacies. If you were cut with legacy status it probably means that you do not have much in common with the guys in the house. Therefore I would strongly advise to look at other fraternities. By now you've had some time to meet guys in other houses, so go where the guys share your same interests, personality, etc.
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