I believe the reason is a holdover from the old days, the traditional view that the purpose of marriage is to have children and that everyone wants to have children. Basically, the view was "what normal young guy would give up having kids to marry an old woman". If a 50 year old guy married a 25 year old girl, that was fine because she could still "give him children". If a 50 year old woman married a 25 year old guy, she was (this is prior to the creation of fertility treatments) beyond her child-bearing years and couldn't "give him children".on-hold said:For whatever reason, it is considered more 'normal' for an old guy to marry a young woman, than vice versa
In Western countries, it's now acceptable to not want kids and also possible for a woman to have children much later in life, so it's not really a strange thing anymore to see older women with younger men. But in many cultures and countries around the world, not wanting children is still seen as very abnormal, so a man giving up his chance to have kids to be with an older woman is seen as abnormal. Anything that is seen as abnormal within a culture is a red flag for CIC.