Uh, because they LIKE to put it there?
Two letters.
K Y
How about a person with low self esteem gravitates towards an area where he/she feel they could excel. For a boy, he would be weak and would gravitate towards girl things for acceptance. For a girl, she would be strong and would gravitate towards boy things for acceptance.
As they get older the traits they developed take hold. For a boy, he spent so much time around girls that he also wants to date a boy. For a girl, same thing. She has developed traits that are male and would want to date girls.
Inevitably, they gain self confidence and for them being gay seems normal.
No government grant, no research study, just an opinion which probably makes more sense and cost a lot less.