It's a number of things.
Lack of role models is a big one. The youth coming up that realize their sexuality don't have anyone to emulate or to learn from. Part of that is really that the gay culture doesn't have many big names to choose from. There are some that are coming up and are making real names for themselves but not many.
Hate speech and bullying are also major disadvantageous. Kids learn young that being gay sets you apart and makes you someone who is hated. What's more in many places the school system has legitimized that feeling by making it against policy to correct students that area bullying gay students. "Neutrality" policies have made teachers fear correcting, stopping, or intervening, on the bullying of gay students for fear of it being interpreted as "encouraging" homosexuality. It's basically become okay to bully gay students in some places and that's just sickening.
"The propagandit's purpose is to make one set of people forget that certain other sets of people are human." -Huxley
"The right to marry is a fundamental right inherent in the liberty of the person, and under the Due process and Equal protection Clauses of the Fourteenth Amendment couples of the same-sex may not be deprived of that right and that liberty." Obergefell V. Hodges