Unsure if this may be the reason, or part of it, for your feelings, but I think when we know our parents and grandparents they are older, and we often admire and respect them for being older and wiser, however they had lives before we were born, and made mistakes of course.
I think we are generally not happy to know about their mistakes especially if they were related to issues of illegitimacy. Though it is virtually meaningless today, we know that in the times of our parents, grandparents and great grandparents etc, it was a big deal and anyone who was illegitimate or who gave birth out of wedlock was often ostracised.
I think this is in the back of our minds even today, and some don't like to accept the failings (as it was seen at the time) of our closer ancestors.
Good luck anyway, and I hope your half uncle is a lovely man and that you all get on really well.