If you are a young woman dating an older man, you are a gold digger. If you are an older woman dating a younger man, you are predatory. If you are a dude doing the same thing, you are generally considered 'lucky' if dating a younger woman or tricked into it if dating an older woman. And we all know that any woman is 'asking for it' to begin with. Ah, culture.
There is also an idea that young men need to play and "sew their wild oats" before settling down, where as women should go straight from courtship to marriage. That means that a younger woman dating an older man is acceptable, and getting that financial and physical male protective 'security' that she apparently needs to survive in return, but adds to the idea that a younger man dating an older woman is having something taken from him.
A single woman is sad and lonely. A single man is living the life, yes?
There's the idea that women are mainly around for child bearing (hence, younger woman=good), that younger men are more immature, that men should provide for the family and that an older woman won't be able to do that, etc, etc. Basically, the discomfort around it reminds me of the same sort that dictates that "women shouldn't be allowed in combat because men don't want to see them hurt".
Oo, then there is also the idea that two men in a relationship are gay, but that two women in a relationship are 'confused' and 'in need of a man/are man-haters" as women 'can't really be gay'.
Basically, I let myself and the person I am with dictate the relationship, because culture is a really poor match maker.