There are two reasons. First, every child needs to see him or herself, his or her own culture and gender and life experiences reflected at school, in order to feel a real sense of belonging. A sense of belonging is a prerequisite for learning. And studies have shown that kids whose families are involved at school are more apt to succeed academically. That is the bottom line.
Every child is at risk of being ostracized with anti-gay slurs.
Unfortunately, embodying any gay or lesbian stereotype qualifies a child for abuse. As does having two moms or two dads. Then he has a terrible choice to make. He can lie and walk around in shame or he can be truthful and walk around in fear. In either case, the harassment is likely to escalate to violence if unchecked by an adult.