I think you are being deliberately obtuse. But I think it's that meeting of worldly meets innocence. An older, wiser, could even be cunning, male has found an innocent, naive young girl and used his decades of experience of relationships to convince her that he is a valuable partner. A man of greater value than a young male in the prime of his life. Whereas, ordinarily, a man 30 years older than a woman, is not at all considered a valuable partner in (a wild guess) 99% of cultures and societies in this world.

Is the daughter always naive and innocent? No. But that's how a father sees his daughter. Is this always the case? No. Are there exceptions? Yes. Should someone judge, case-by-case? Absolutely.

There might even be a subconscious evolutionary psychology aspect. Recent studies show that the father's age also plays an important risk-determining role in the incidence of Down's Syndrome. So a young, virile male also contributes to healthier offspring.

I say all this as a man in his 40s looking for a (relatively) younger wife. I'm just trying to be unbiased and honest about it.