Even accounting for that, the vast majority of people are straight
Not true. Over the past years the number of people that identify as LGBT has been growing and it's not because there's more LGBT people, it's just that more people are comfortable with saying who they are.
Hundreds of years ago the Catholic church used to hunt and burn people for being Gay so a lot of Gay people would say they were straight. As time went on and society has become (somewhat) more comfortable with Gay people, more people have decided to come out and identify as Gay.
It's still legal to murder people for being Gay in certain countries. In Russia, "promoting" anything to do with being Gay is considered illegal and women like Lady Gaga and Madonna, people who talk about LGBT rights were putting their lives in danger by even just going to those countries. In America, it's legal to fire someone for being Gay. Trans people can be denied housing.
So yeah...there's not necessarily "more straights" it's just that the LGBT community is so oppressed so a lot of closeted people don't identify with their true sexuality or gender because they know identifying and living life as a straight/cis person is easier.