Those who would find against abortion as not being in the Constitution which most scholars agree with will likewise find "marriage", polygamy, same-sex marriage is not there either.
Hence, these laws should be governed by the states.
So, since Trump has made a point of saying he would nominate "constructionalists" to the Supreme Court, I just don't think he felt he needed to make a big deal about this issue.
One of Trump's supporters is Huckabee, who is very anti-SSM.
Constitutional Amendments are not needed in my view, just allow the state to run it as the Constitution allows.