Straight seems to be the default because that is what your parents except you to be. Most parents want you to come out attracted to the gender which they think is appropriate.For example if you are a boy they want you liking girls and if your a girl they’d want you liking boys. They grew up in an era was mostly everyone was homophobic. So when their son or daughter comes out that they are gay they find it unacceptable. That’s why most people consider straight the default.