Dirty old men and greedy young women

When people see relationships where the man is much older than the woman, they often suspect that both partners are there for superficial and unseemly benefits; money for the woman and a sexual object for the man. If a young man was with the same young woman, why does it reflect less badly on his motives? If an older woman marries an older man, why is it less plausible that she is just after his money? Why does trading one of two superficial motives for a relationship – the man’s youth or the woman’s money – and replacing it with a different superficial motive – the man’s money or the woman’s youth – make the relationship more likely to be superficially motivated? If people really fell in love for non-superficial reasons, shouldn’t we expect to see more couples who don’t match on superficial criteria such as age?