Voting to ban same sex marriage is immoral.
What is the definition of morality?
Voting to ban same sex marriage actively HARMS other people by refusing to allow them to have civil rights. Banning same sex marriage means homosexual people cannot form families. That is one of the cruelest things one group of people can do to another group of people, because the family is the foundation of our society.
People engaged in homosexual relationships are not doing anything immoral because they are not harming anyone. They are consenting adults. It is the anti-gay people who are committing immoral acts.