Birth rates in the US are plummeting. They peaked in 2008 at 2.1/woman(the replacement rate) and are now down to 1.7.
I can only offer anecdotes but I know several people of prime child rearing age who don't have families. From my discussions with them, they don't feel secure financially. Maybe people raised in a middle class environment aren't willing to risk poverty to have kids?
Or maybe it's the endless distraction we have available to us. Or maybe it's the increasing isolation of modern life. If you weren't raised in a coherent household maybe you feel less of a need to create one yourself.
One of my coworkers was so desperate when she hit her late 30s that she stopped taking birth control and had unprotected sex with any guy that was willing until she got pregnant. Whoever the guy was, he has no idea he fathered a child. She just wanted him for his sperm. The kid is basically being raised by child care centers while she continues on her career path. The other female coworkers think she is smart and are planning on doing the same thing if they find themselves in a similar situation when they reach their late 30s.
That's just unethical. I feel sorry for the child already...
Sleeping with some person doesn't tell you anything about, say, their family medical history. Sperm banks check that stuff.
However, the explanation that makes the most sense to me is that it is a lot easier for a guy to seriously say, "I want kids," then it is for a woman to seriously say, "I want kids." Being a sperm donor is a heck of a lot easier than actually going through the pregnancy. Even after the kid is born, in a lot of places, including where I live, the majority of the child-rearing responsibilities fall on the woman by default.
Have you seen the price of housing lately?
Have you seen the lack of /careers/ that used to make living in a house for along time a viable idea?
One idea that might work is to give up on trying to get young people to use birth control. Everyone has a number of kids in their late teens/early 20s. Child-rearing would fall to the ((great) grand)parents, who are more financially/socially stable and potentially retired. This has a number of substantial benefits, including noticeably better medical outcomes for both mothers and children.
I think the earth would be much better with a target population closer to 1 billion. That way everyone could have a high and worthwhile quality of life without killing the environment.
Until we reach that goal, a global target of each adult having the birth right to produce half a child is fair.