Trump was pro-gay marriage. He was actually the first President in US History to be pro-gay marriage before winning the position.
I do wonder where Trump as being against gay marriage came from. My Mom, as a life-long Republican who lives in Utah and detests Trump, also said his stance against gay marriage was part of why she hated him. That false narrative seems to have drown out the truth. People who are pro-gay marriage hate the most pro-gay marriage President as compared to all other Presidents in US history, many of which I'm sure they supported, because they're convinced of the narrative. It blows my mind how well propaganda works.