[vannevar]

That's a general social trend, not unique to the left. Steadily increasing numbers of Republicans also support gay marriage, currently around 40%. (http://www.pewforum.org/fact-sheet/changing-attitudes-on-gay...)

[ZeroGravitas]

The same source shows a roughly similar shift on immigration, with older people being less keen and the average opinion changing as they age out:

http://www.people-press.org/2017/10/05/4-race-immigration-an...