I don't think so, I mean guys are physically uninteresting to me. I can't see why I should be attracted to someone who is physically similar to myself, seems boring. Never had a boner that was caused by a guy
I agree with Shaun. If the subject of the arousal is the other male itself, then there is more than likely some sort of bi-curiousity there. However if the arousal is more situational (relaxing on the beach, enjoying a breather after the gym, etc.) then I'd say less so. There are many occurrences where there simply is no reason at all for the arousal and it just happens, hope this helps :)
No. Yes that boner would make them bi or gay. But if you're asking because a guy got one around an attractive guy... keep in mind that sometimes some guys just get random boners, boners from vibrations, sometimes from their junk rubbing just right against their jeans or something. So don't jump to conclusions too fast!