THIS.
Is fear-mongering.
Gay marriage was voted on by the Supreme Court, and, even with Trump choosing three SCOTUS judges, Presidents cannot overturn the SC’s rulings. The SC takes overturning previous rulings VERY seriously. It’s not something they’ll do all willy-nilly. Someone will need to come forward with new evidence and plead a new case, and, while it can happen, I highly doubt it will.
Gay marriage is safe, Trump couldn’t touch it if he wanted to.