"I don't see why a mild preference in favor of either sex to achieve "gender balance" should be disallowed."
It isn't a 'mild preference' any more. If they admitted women vs. men strictly on GPA and test scores, with the exception of STEM schools, the top 100 ranked colleges in US News rankings would skew something like 70% female/30% male. Boys are being given a huge break in admissions these days. So anyone who wants to get rid of "preferences" and go with strictly score- & GPA- based admissions needs to accept that as an outcome. As soon as you say, well... that isn't so great, we should give boys a break, then you have to ask why any other underrepresented group should not also get some breaks as well. You can't have it both ways.
I don't love the donation admit, either. But gotta say that enough money to build a new library or a new science facility that thousands of students will benefit from over the years is worth something to the school AND the student body. The number of those types of admits are ridiculously small, too.
At most colleges, alumni kids who truly aren't qualified get a "courtesy waitlist". One of my kids, who would have been the 8th family member over four generations to go to a top ranked university got one of those. And honestly, they were right to do so (not admit her). If an alumni kid gets in, it is because they met the qualifications to be seriously considered, then got an edge because of their alumni status. Most colleges no longer let the kid in if they don't stack up to the rest of the admission pool.