The first president to go to into office already supporting gay marriage was a republican (his name was Donald Trump).
https://static01.nyt.com/images/2018/10/23/opinion/23bruni-n...
https://static01.nyt.com/images/2018/10/23/opinion/23bruni-n...
The 2016 and 2020 platform both said that marriage is between 1 man and 1 women, they didn't mince words. Pretending Republicans won't try to overturn gay marriage is about as stupid as the people who said they would never overturn Roe v Wade.