Do you really believe that's the universal reason? Because that's not why Abraham took Hagar. He took her because his wife advised him that he could get a son, his desire being to open himself to God's blessing of a heritage. Not even just a son, but a whole lineage of children.
In light of this, I find it to be presumptuous to assume what other people's motives are in the choice of a spouse. But, I am not picking a fight with you.
I spent my teen years listening, talking to and learning from adults rather than socializing easily with other teens, although I did dabble in a homeschool culture and made a few friends. I was gravitating constantly towards a type of environment and thought process that made finding a mate seem like an impossible task - especially when looking at the minds of younger men.
At 18 I was painfully lonely. I needed someone to love and I had to draw on an older pool of males to even find a man who could relate to me and deal with me. I needed someone who was at least 30. The men younger than that seemed like children to me.
But I'm different than most women and that's not a brag. (it would be a pretty sad brag) It's a socially isolating fact. I think like men, and not any men, like older men. Even moreso now. In the past 15 years of marriage I found myself geeking out on tons of scholarly articles. It's more fun than reading the news and everyday I have more questions leading to more research.
Thankfully my husband can tolerate doing the research with me and talking about it with me ad nausium.
It helps me feel connected with someone mentally.
The perk is he gets an "old man" in a young woman's body, capable of bearing many children and loving being a companion who stays home, too.