We don’t need to amend the Constitution because the Constitution does not address marriage.
We do need to have a president that will only nominate Supreme Court Justices that follow the constitution, not make it up as they go.
Originally, it didn't address slavery, woman's right to vote, prohibition, etc. It is why our Founders included the ability to amend the document. We can define marriage in an amended constitution. Many states amended their constitutions to define marriage as being between a man and a woman. The Supreme Court overturned many state constitutions on the issue.
We do need to have a president that will only nominate Supreme Court Justices that follow the constitution, not make it up as they go.
We may wait a long time before such Justices are in the majority. We still have Roe vs Wade, the Kelo decisions, etc. The tyranny of the courts is a problem.