As a millennial, I will say there are 2 reasons for this.
1. We want more social freedoms. We believe the government has no right to dictate our lives. My body my choice, both ways. Do drugs, not my right to tell you no. You are gay? Cool, doesn't matter. Marriage is a religious institution not a government responsibility. Tax implications are another story and if two people are “married” the federal government should not be regulating morality. This is where Republicans mess up. Stop focusing on these petty things and focus on what matters. (THE ECONOMY, FOREIGN AFFAIRS, SPENDING ON OTHERS WHILE WE STRUGGLE, ETC)
2. Indoctrination is a thing and it is getting worse. When I was young I was told by a student teacher “democrats are for the people and republicans are for corporations”, BUT IT WASNT TGROWN INTO MY CARTOONS. This has been running on Nickelodeon, over and over for at least a week.
https://youtube.com/shorts/_HyZScYhL0I?feature=share
If more freedom is what you millennials want why is your preferred party so much more authoritarian than it was fifty years ago?
You are confusing licentiousness with liberty. Licentiousness will lead to your enslavement.