There are a lot of factors involved with this, some of which the others have touched upon.
The biggest one is by far the invention of female contraceptives (birth control, etc.). This has basically allowed women to choose when and if they have children.
When this technology emerged, we also start to see women pursuing college and careers. There's nothing necessarily wrong with this, but it ultimately leads to women postponing marriage, children, etc. The mothers of the boomer generation were already married AND had children by the time they were in their early to mid 20s. In fact, it wasn't unusual for women to be married at 18 lol. It also was the norm to marry the first man they had sex with, were dating, etc. In contrast, boomer women may have dated and had sex with multiple men before getting married to their husbands, having children, etc. This causes some profound changes with pair bonding for sure.
Though this wasn't as common in the boomer generation, younger generations of women are often heavily pursuing high levels of educational and career attainment. This further delays the process of marriage and children, often not even being remotely on their radar until they are well into their 30s. The other issue that this has is these women become increasingly pickier and demand a man of at least their own "value". This renders their dating pool much more limited and when you combine this with them being over 30 it's highly likely they will not get married. I saw a stat that if a woman is unmarried/single AND childless at 30 she has something like a 50% chance that she will never have kids.
I also believe that there is overall much less societal/culture value placed upon marriage and children today compared to what you would have seen with previous generations. A big part of this I personally believe is that many millennial and gen z individuals, both men and women, grew up in divorced or single parent households. It's totally easy to see why someone like this may be hesitant to get married. Many divorces are/were honestly quite intense and I believe could emotional scar the children that are caught in the middle of it.
Getting the female equivalent out of the way.
Economic factors are also a huge part of this. Back in 1950 a man could support his wife and children on his income alone. Over the decades, this changed to where both husband and wife may have to work. This leads to family planning, only have 2 kids, etc. More recently, we are seeing many adults struggling to get by on their own. Imagine living pay check to pay check to barely scrape by, or having to pick up a side hustle or part time job to make ends meet. Dating and relationships go on the back burner for sure.
@Solomon posted some article about how people aren't even able to afford a simple date and needing to go into debt for it lol. Let's say you are in a relationship, any sane person wouldn't go and have a child when they are in this spot financially lol. Side note, I legitimately don't understand how the poor and underemployed class gets by, let alone when they have a bunch of children.
I partially wonder if this is all by design to depopulate the planet OR just the side effects of the very rich trying to milk everyone or everything they have.
