Just because something is an acquired behavior shaped by environmental factors does not mean that it is easy to decondition or undo. Look at addiction, for example. There are people who are addicted to gambling, pornography, video games. Those addictions are often extremely difficult to break. And yet, they are entirely environmental. Before video games were invented, there were no video game addicts. Environmental conditioning does not imply someone waking up one day and deciding that they are going to try being gay for a change. It includes things like being sexually abused as a minor, growing up without a proper male model, and other types of extreme emotional trauma.
A study that I would really like to see is one that measures the percentage of children adopted by gay couples that grow up to be gay themselves. I would venture a guess that it is way out of proportion compared to children raised by heterosexual couples. But no one would date to conduct, let alone publish, such a study.