My opinion is based on anecdotal evidence... But I think we're still feeling the societal effects of access to birth control. (Western) Women have had a choice about whether or not to be mothers for a relatively short amount of time. Only a couple of generations.
My maternal grandmother (came of age during WWII) wrote in her personal diary that she wasn't excited about motherhood, but felt it was her duty and that her husband was excited to be a father, so she was pleased that he was happy. She then suffered from serious post-partum depression for which there was no treatment.
My paternal grandmother also struggled with poorly-treated mental health issues. When my father was 5, she told him she really didn't enjoy being a mother, so he was going to have to step up and be the man of the house while his father was at work and take care of himself and his little sister.
If either of those women were in their 30's today, they likely never would have chosen to have children, but there was no choice for them - not only because of lack of birth control options, but because culturally there was an expectation.
In my maternal line, I have an uncle who chose to remain childfree (long term heterosexual partner, but no marriage), and my mother admitted she felt ambivalent about motherhood, but that it seemed like the next step for her marriage/relationship. She had access to birth control, but no friends or family who modelled a child-free lifestyle (her brother was younger, so he hadn't really committed at that time).
In my paternal line, I have an aunt who also had two children, but basically admitted the same feelings: that if she were one generation later, she would have pursued a music career instead of marriage/children - she became a music teacher and is still married. Neither of her daughters are having kids and neither is married in their late 30s: one is a doctor, one is a musician.
I have two siblings. None of us will be parents. One of my brothers is an inheritor of our family history of mental illness and chose to get a vasectomy in his 20s. Myself and my other brother are both married and neither of us have ever seen children as a net-positive.
That general ambivalence toward parenthood that lurked in our grandmothers' generation has turned into true choice now: We aren't unusual in choosing not to have kids, and we have plenty of access to birth control.
If my husband dies or we divorce, I would not choose to marry or cohabitate again. I'm beyond the age where children are a question, I'm financially secure - marriage brings nothing to the table for me. And cohabitation with a man who is 50+? I love my husband and all, so he can stay, but other dudes can keep their CPAPs and lifelong habits and personality quirks in their own house. We can hang out, go on vacation, have sex... but I don't need that 24/7 in my house.