I believe the real major changes began during the 1960s hippie era of "free love". The "baby boomers" began pushing the envelope with the advent and popular use of birth control pills.
Swinging/swapping along with what was called "shacking up" came into vogue. Essentially "pre-marital sex" became a normal dating practice. This was probably the (first) major dating change!
The next major changes came during the ERA movement. Some women took to streets to burn their bras in protest. Although the amendment failed to be ratified better career opportunities and higher incomes became available for women. This gave women options that previous generations of women did not have. There were more women financially capable of taking care of themselves which allowed them to delay the pursuit of marriage.
Some women completely welcomed the newfound freedom to be on par with me and even chose to emulate sexual behavior that was often ascribed to men. Virginity also decreased in value.
Better birth control reduced odds of unplanned pregnancies.
The 70s brought the Disco era and "one night stands" were fairly commonplace in the nightclub scene. Female entertainers and especially pop singers of the 80s to now really started to push sexuality in their songs, the way they dressed, and behaved in their videos from Cher, Madonna, Britney Spears, to Beyoncé , Nicki Minaj, Rihanna, Jennifer Lopez and Iggy Azalea "Booty video" have made it cool and empowering for women to be seen as sex symbols. Their young teenage girl fans became more sexualized.
Jelly bracelets aka sex bracelets started being worn by teenage girls and the various colors indicated to boys what they would be willing to do. •Yellow - hugging •Purple - kissing •Red - lap dance •Blue - oral sex •Black - intercourse. The media picked up on this in 2003.
It's not uncommon for teens 15 years and younger to be sexually active.
Online dating and social media has increased the number of options people have to meet other people. The more options one has the less likely they will be in a rush to "settle on one". They want to see what else is out there!
People do still want to commit and they will commit however the "timing" has changed. Neither gender feels any urgency to marry or commit at age ranges previous generations considered to be "the time" to settle down. One man's opinion!:)