I don't think it's a case of gay guys falling for straight guys.
Feelings are feelings, and they cannot be prevented whether we want them to be or not. There are girls who fall for guys they will never have, and there are guys who call for girls that they will never get. When you develop such a strong relationship with someone and feelings are accumulated sexual orientation is irrelevant.
There were straight guys I fell for, and it was not because "I thought I could convert them" or "I liked wanting what I could not have" it was more so these guys would treat me so well, and we would have such fond memories together that it was impossible to not fall for them, when all they were being is a genuine true friend.