It is hard to believe that some people say that being gay is a choice. It is something that people are born with, like the quote says, it isn't voluntary. But, what people can do is stop spreading hate and instead just allow these people to be happy.
The shirt originally said, "Some people are gay. Original pin: Get over it." They were made as part of a demonstration for gay rights in the UK. Sir Ian McKellen has been an outspoken advocate for gay rights.