EDIT: One thing I forgot. Some say that even women are having less sex than before, though not as much as men.

The primarily cause of singleness among men seems to be social atomization, which has been exacerbated by the growth of technology and the decline of real-life third spaces.

As a result, people are more asocial than ever before.

For example, even at places like bars, one example of a third space, individuals can easily choose to ignore those around them by turning to their phones. In contrast, before the Internet, people didn't had that choice so they were more "forced" to engage with each other IRL.

This has made both dating and building friendships [men with more friends get laid more] more challenging, and some argue that people today have fewer friends than they did in the past.

Single men tend to struggle with not only finding romantic relationships but also forming friendships as well.

Fundamentally, it doesn't seem that men and women changed all that much, but the social landscape has shifted significantly.