Although I wouldn't date a 20 year old girl (I am 40 and married anyway) simply because of the maturity difference and because I already have children, I certainly wouldn't say it is "gross" to be with someone simply because they are young enough to be the person's daughter.
You do realize that could mean just an 18-20 year age difference (or less in some cases), right? This would mean that a 58 year old man couldn't date a 40 year old woman without being "gross". I don't think so.
Furthermore, and of significant importance here is that throughout ALL of recorded history (until JUST RECENTLY) in nearly all cultures it was considered VERY normal for 30-35 year old men to marry 13-16 year old women. In fact, this happened all the time and was widely accepted everywhere. It would have been considered "weird" for a man who was well off and respected in his community to marry a woman his own age. In most cases the man was AT LEAST 10-12 years older...minimum.
Why were things this way? Back then the men "took care" of the women--completely--and that wasn't possible (for most) if the man was only 20 years old, as he hadn't yet acquired the means for doing so! Also, one of the main considerations when marrying a woman was her ability to bear children. A younger woman (in her young to mid teens) was considered ideal because she was not only more likely to bear healthy children, but she could have more of them and be less likely to die during childbirth.
If a 35 year old man married a 35 year old women, he could basically say "goodbye" to a family, as she would not only be nearing menopause, but she would be less likely to have healthy children and more likely to encounter health issues--and die--during pregnancy due to the lack of medical technology. And with sickness and disease claiming WAY more lives back then, having children die young was very common. Basically, you needed to beat the odds in order to produce a family, which meant having a wife young enough to get pregnant many times...because if your wife was impregnated 10-12 times (assuming she lived through all of those pregnancies), 3 of them might result in children that actually grew to an adult age. Furthermore, the average life expectancy was way lower--like 45-50! If the mother was 35 when she got started, she likely wouldn't even have been around long enough to raise her children into maturity! This is why, if a woman wasn't married by her mid-20's (late 20's at the very latest and that was pushing it), she would likely remain single for life.
There are other reasons as well (also supported by science), but it is anything other than "gross". Only someone with a limited perspective would see things that way. If anything, it is today's culture that has a skewed perspective...not the other way around.