I get that him grabbing your butt was completely inappropriate but do you really need a therapist over this? It sounds like the two of you have had a great relationship prior to this and he is probably an alcoholic.

You were right to confront him but given the context was it really so traumatising you need therapy and are interested in ruining relationships with your family? I guess (assuming there's not more to the story and assuming you don't think this is something he would do again to either yourself or others) I don't understand what your end game is in holding on to something that was obviously a drunken mistake.

If it were me I would forgive him but not forget his actions and watch out for red flags + minimise contact with him. If there's anything you can do to encourage his sobriety do it. He may well be a good person who has a serious substance abuse problem and that spiral will change him from your loving Uncle to something else.