While the issue that you've described may seem simple and straightforward from the point of view of a straight man. There is something that you are not taking into consideration, probably because you are not aware of it, i.e. there are a lot of men on dating sites who state that they are straight but also play with guys on the down low. They will not state they are bi as they believe it would significantly affect their chances with women.
Any suggestions on how to tell the difference between between someone who says he is a straight guy and another guy who says he is a straight guy?
If there was a 100% reliable way of finding out who is interested and who is not, I would have no interest in contacting a straight guy who is not interested.
And the assumption that the majority are not interested is erroneous as when a guy hasn't been with a woman for years, the chances of him considering other options inevitably increases.