The increase in social liberalism has not been universal, but has occurred mainly among Democrats. The percentage of Democrats describing themselves as socially liberal increased fairly steadily from 36% in 2001 to 53% 2015 where it has since held.
The increase in social liberalism in the U.S. seen since the early 2000s is the result of increasing liberalism among Democrats, and particularly among white, more-educated and older Democrats. The changes by age mean that various age groups of Democrats are now in greater political alignment. However, the changes by education and race have widened the divide on social issues between Democrats with and without college degrees, as well as between white and black Democrats.
This doesn't necessarily mean Democrats are at odds with each other. Indeed, despite the widening gaps along race and education lines, 89% of Democrats supported the Democratic Party's nominee for president in 2016. However, as Democratic leaders debate how to redefine the party post-President Barack Obama these data suggest that moving any further to the left on social issues could risk alienating Democrats with lower levels of education.
Those are the kinds of voters President Donald Trump might try to attract in a second-term bid, particularly if his GOP base is faltering. On the other hand, with most of these lesser-educated Democrats describing themselves as moderate on social issues rather than conservative, that would be a hard sell.