1. somewhere between 5 and 20 percent, I'd wager.
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http://en.wikipedia.org/wiki/Demographics_of_sexual_orientation
It's a very hard guess. Though based on personal experience, if someone is definitely hetero, they recognize that it is members of the opposite gender that drive them, sexually. If the straight man, when under pressure, admits to having a slight curiosity, it is either to test himself, or to experiment. I would not say that he is variable in his orientation.
2. the 'gay' is probably deluding himself in the same way as anyone else who has a crush. It is hope with a small chance of 'success', but I would say his actions should depend on the receptiveness of the hetero male. Push for one's desires, but be respectful when another's desires are concerned.
3. I do not think there is any struggle here on the part of the hetero male. As stated above, hetero appears confident of his orientation. Further pressure by the 'gay' will probably be detrimental to the current relationship.
To the hetero: It is clear your friend wishes to advance your friendship. Be clear with your own intentions and set boundaries if necessary; or grant permissions if you decide the other way. Communicate.
To the 'gay': it is okay to be attracted to your friend. It is not okay to pressure your friend into a relationship he may or may not want. Simply discuss your motives/intentions/desires. Be ready for either outcome. Support your friend's choice, just as you would want him to support yours.
To both: respect. communicate.