Is it a sin to put someone down that is gay?
Some people think that being gay is a sin and many people will make fun of or ostracize people that are gay. What they don't realize is that by making fun of anyone they are participating in something that is wrong and they should repent and change. What would Jesus do is a good test question if you are wondering whether you should say or do something to someone else because of their behavior.
Answer below comes from a Mormon Apostle - Elder Oaks. This is out of the middle of a talk called Protect the Children from October 2012 LDS General Conference.
When we consider the dangers from which children should be protected, we should also include psychological abuse. Parents or other caregivers or teachers or peers who demean, bully, or humiliate children or youth can inflict harm more permanent than physical injury. Making a child or youth feel worthless, unloved, or unwanted can inflict serious and long-lasting injury on his or her emotional well-being and development. Young people struggling with any exceptional condition, including same-gender attraction, are particularly vulnerable and need loving understanding—not bullying or ostracism.
With the help of the Lord, we can repent and change and be more loving and helpful to children—our own and those around us.
This is found in the October LDS Conference of 2012 Elder Oaks Protect the Children -