Here's why: The president doesn't themselves decide these kinds of things. Marriage equality, as it is nationally, was established by a Supreme Court case in the summer of 2015. The radically far-right judges Trump has listed for potential nominees for the Supreme Court vacancy, as well as any vacancies in the future, are appalling. And it looks like there's a good chance those nominees would be approved, because the Republicans hold majorities in the Senate and the House. Draw your own conclusions, but it looks like marriage equality is in danger. And this is if we are generous enough to assume he actually does support it in the first place, which I doubt.

As an aside, Trump's VP pick alone is pretty indicative of his stance on this too. Mike Pence has advocated for conversion therapy for LGBT individuals in his previous office as governor of Indiana. Need I say more?