A woman's role

Even if it's subconscious, there is a general assumption that women are made to have kids and that you don't do your womanly duty you're breaking some sort of code. If you go against the grain, unfortunately old, outdated ideas still persist.

In the natural world, males instinctively seek out a young partner in order to procreate and continue their descendent. Humans are essentially the same, which is why seeing a younger woman with a sugar daddy doesn't strike us as strange - especially as they often have kids very early into their relationship (whereas with a younger man the reverse is often true).