I can write an entire essay on why it is ok and why people who say it isn't are just narrow minded bigots, but I'll keep it short.

There is no logical reason why it is wrong for a 45 year old to date a 20 year old. The reason people have a problem with it is the same reason people have a problem with homosexuality. A lot of it also has to do with people's automatic assumption that women are weak and fragile and men are always looking to take advantage of them. Most people who would hate a 45 year old man for dating a 20 year old girl would not have the same problem with a 45 year old woman who dates a 20 year old guy. They will probably claim they do just for the sake of not being sexist, but they don't. Many dads will openly admit that they would never want their daughter dating an older man but would be ok with their son dating an older woman. Double standards are a pretty good indication that a person's logic is based on prejudice and ignorance.

Sorry if I hurt anyone's feelings, but quite frankly I think that holding these prejudice views and judging people based on them can be pretty hurtful to those people. So maybe you should rethink your position.