Don't take this the wrong way, but you must be young or not following media 20 years ago. Trump was outspokenly for gay marriage since the early 2000's. Trump is literally a 90's Democrat in attitude, actions and personality. Trump has always been incredibly Laissez Faire on social issues. Let it sit in that almost all of his policies would of been common Democratic values in the 90's: Tough on crime, Strict on the border and even donated a decent chunk to the DNC during those years.
https://www.pinknews.co.uk/2016/03/05/donald-trump-celebrated-elton-johns-marriage-in-2005/ (You don't have click the link to see that Donald Trump supported gay marriage since
2005. )
Trump even donated to Black causes and hung around Black celebrities since the 80's, when even "liberal" Hollywood left them alone. The only reason that Trump even took 2016 seriously was because he hated Obama's policies, hated that the DNC started to take for granted his donations and getting made fun of by Obama on stage helped his drive and he REALLY didn't want Hillary Clinton to be president.