There was an episode on HBO's Entourage where a str8 dude (I think his character is named Kevin) was doing nice things for his str8 (former college football star) massage therapist. He bought him a present , was very complimentary about his skills as a football player, and invited him to a party. Anway, at one point, Kevin ended up in his hotel room for a massage because the spa area at the hotel closed before he got his massge. The therapist jumped into bed when Kevin went into the bathroom, he was all confused as he did this. You could tell that he wasn't sure if he should do it or not. When Kevin came out of the bathroom, he was shocked and embarassed. "I'm not gay, dude", or something like that. The therpaist said he wasn't either, but he would do it because it seemed so important to Kevin and he had been "so nice".
I found this story sorta hot even though it was fiction. The str8 massage therapist was willing to have sex with Kevin because Kevin had innocently flirted -- like friends sometimes do -- by telling another dude how good he is at something, trying to include him in your own social plans, smiling and having fun with him, and ending up in a ambiguous situation (i.e., alone in a hotel room).
I admit I had this situation on a business trip one time. My male co-worker (who was str8, married, and good looking) had spent a week with me at a seminar. We had a blast going out for drinks and doing some site seeing, etc. in the evenings. Our last night in the hotel room, he leaves the bathroom door half open when he was taking a shower before bed. It was one of those showers with a clear glass door, so I could see him totally naked in the shower -- a very LONG shower. I almost walked in and got in the shower with him -- but i chickened out. At bed time, i thought of getting a hard-on and making a joke about needing to beat-off after a week without -- but i chickened out. The massage guy had more balls than me, but he ended-up reading the signals wrong. It was still hot.
In the end, I think men (str8 ones, anyway) are wired to want sex all the time. If someone we like (female or male) sends us signals that they want to have sex with us, I think it can happen.