In some cultures where there is segregation of boys and girls from early childhood, the boys end up being with boys because of unavailability of girls and then they end up liking it so when they do grow up and get married to women, they still continue doing this on the side.. But then in the west, there is no segregation whatsoever between sexes and women are freely available and yet there are gays there out of free choice.. So you can't really say what the reason is.