There's less incentive for both men and women to marry. Here are the reasons...
1. Options
As former red pill blogger Chateau Heartiste used to say, Options = Instability.
How you experience options depends on where you are in the social hierarchy. Women of all sorts have the most options. There are been swipe app experiments done where overweight/obese women got more likes in a given 24-72 hour period than top tier men. If you are any woman or a top tier man, the options are great. If you're a mid-tier (normie) range male or a below average male, you have fewer options and even fewer options than the mid-tier and below average men of 20-40 years ago. I'll start with discussing men's options and then move on to women.
For top tier men (alpha/sigmas, often called 'Chads'), the options are more plentiful than in previous times. With swipe apps and Instagram, a top tier man has more access to more women than he had in the past. In the past, even top tier men were confined to what they could get with their in-person activities. They could get women in the nightlife venues when they showed up there or in some non-bar venues. Now, a top tier man using a swipe app and being active on Instagram has far more options than even a top tier man who was active in 2000 could have had. Being on a swipe app is like being in 10 or 20+ nightlife venues simultaneously and 24/7.
In the past, a top tier man might get tired of all the nightlife venue visits and daygame and eventually select a top tier woman for a longer term relationship. There are still some top tier men that do this, but they might do it now many years later in life than they would have done it in the absence of swipe apps and Instagram. Now, a top tier men can have sex with top tier women (often called 'Stacys'), more average/slightly above average women (often called 'Beckys') or even subpar women. The options are endless, but most top tier men who choose to run rotations have women who are at least average looking.
More average tier men (normies) have fewer options than before. In the past, mid-tier men would fail to seduce top tier women and eventually end up with some more mid-tier women. Now, with the mid-tier women (the 'Beckys' of the world) chasing the top tier men more actively, a lot of the mid-tier men end up with very little. A mid-tier man without a good social circle giving him access to mid-tier women could end up dumpster diving for a lower tier woman on an app or possibly in a nightlife venue. Even more commonly, a mid-tier man is turning into porno tube masturbator and/or a pay pig. Most pay pigging is online, but there's even offline pay pig stuff at strip clubs, bikini coffee houses, and breastaurants. Online pay pig stuff is way worse. These occurrences have been why Rollo Tomassi is well known for saying that the average woman doesn't want much to do with the average man.
The below average men can't even get below average women anymore. They are incels, MGTOWs, online pay pigs, porno tube masturbators, etc.
For women, the options have increased across all looks tiers and ages. Even a 30s/40s past her prime childless woman has more options online than a 20 year old supermodel would have had in the pre-internet era only using offline ways to meet men. The increase in options for men encourages them to only devote their time to the best ones (if they lack a social circle) or eventually choose to leave a longer term relationship or marriage for the plentiful, theoretical options that now exist for women. Women in LTRs and marriages now know how many options they'd have if they left their current relationships. Married women and women in LTRs all have friends/acquaintances that have either never married or got divorced/got out of multi-year non-marital LTRs and saw incredible demand in the open sexual marketplace for them. Additionally, most women have also read articles or seen YouTubes/TikToks about all the options that exist out there. This goes well with the second factor I'll identify.
2. Gynocentric social order and family court incentives for divorces
One of the biggest achievements of 1960s era Second Wave Feminism was instilling a gynocentric social order (a term I learned from Rollo Tomassi) into most Western nations. The gynocentric social order and 1980s-2000s era self-esteem boosting Boomer parenting of Millennial women have contributed to why women might not want to marry anything but the best. Millennial women were told by feminists and by their Boomer parents about how they great they were for just existing. They were told to "Never Settle!" and to always pursue the best. The culture encouraged them to get educations to avoid being dependent on any one man. Most White women in the USA believe they are entitled to receive a top tier man and don't want to settle for lesser things. The culture + all their options from technology tell them this.
On the men's side of the equation, men realize that the gynocentric social order has given women many benefits while keeping in place a family court system that still gives incentives to women like it was the pre-1970 era when women were dependent on men. Women can explore all their options if they get bored in a marriage and still get some good financial incentives for leaving the marriage. Why would men agree to such a system slanted against them? The top tier men can stay unmarried and enjoy all their options and the mid-tier and lower men can go MGTOW and pay pig for less than a divorce settlement would cost them.
3. Decline of Judeo-Christian Religions in the West
Judeo-Christian religion practitioners have a way of pairing off earlier in life and staying together as a result of cultural forces. Without the foundation of Judeo-Christianity in the West, reasons 1 & 2 become more prevalent.
In the past, with more people practicing a religion, there was an incentive to marry. A lot of Judeo-Christian faiths ban pre-marital sex so a lot of marriages in the past were done for sex. There were other benefits besides sex in marriage.
4. 3 generations of adults have now seen their parents' marriages fail
In the United States, California initiated no-fault, liberalized divorce laws in 1969 and most other states in the USA followed California's lead during the 1970s.
The earliest divorcees from 1969-1975 were pre-Boomer, Silent Generation members (late 1920s-1945 births) often with early Generation X children. From 1969-1975, most Boomers were either too young to be married or were newly married and yet to be mad enough yet to divorce. Members of Generation X were the first children of divorce and known as the latchkey kids. From the late 1970s-early 2000s, most of the divorcees were Boomers with 1980s-1990s born Millennial children.
Gen X, Millennial, and now early Gen Z adults have all seen the impact of their parents' failed marriages since the 1970s. There are now Generation X and Millennial adults who experienced parental divorces in the 1970s-early 2000s who are now divorced themselves. With the Millennials, parental divorces and the other factors have prevented a lot of marriages from happening. 1980s born Millennials are turning 40 right now and there are more never married 40 year olds than ever.